diff --git a/include/ruby/backward/2/attributes.h b/include/ruby/backward/2/attributes.h
index 7a1ea6b758..916d9e9d5b 100644
--- a/include/ruby/backward/2/attributes.h
+++ b/include/ruby/backward/2/attributes.h
@@ -81,10 +81,8 @@
 #undef NOINLINE
 #define NOINLINE(x) RBIMPL_ATTR_NOINLINE() x
 
-#ifndef MJIT_HEADER
-# undef ALWAYS_INLINE
-# define ALWAYS_INLINE(x) RBIMPL_ATTR_FORCEINLINE() x
-#endif
+#undef ALWAYS_INLINE
+#define ALWAYS_INLINE(x) RBIMPL_ATTR_FORCEINLINE() x
 
 #undef ERRORFUNC
 #define ERRORFUNC(mesg, x) RBIMPL_ATTR_ERROR(mesg) x
diff --git a/include/ruby/internal/dllexport.h b/include/ruby/internal/dllexport.h
index 7f152441a9..71026e7100 100644
--- a/include/ruby/internal/dllexport.h
+++ b/include/ruby/internal/dllexport.h
@@ -37,9 +37,7 @@
  * ```
  */
 #undef RUBY_EXTERN
-#if defined(MJIT_HEADER) && defined(_WIN32)
-# define RUBY_EXTERN extern __declspec(dllimport)
-#elif defined(RUBY_EXPORT)
+#if defined(RUBY_EXPORT)
 # define RUBY_EXTERN extern
 #elif defined(_WIN32)
 # define RUBY_EXTERN extern __declspec(dllimport)
diff --git a/insns.def b/insns.def
index 9f5ee7095a..c6284ee444 100644
--- a/insns.def
+++ b/insns.def
@@ -1345,10 +1345,8 @@ opt_aset_with
         val = tmp;
     }
     else {
-#ifndef MJIT_HEADER
         TOPN(0) = rb_str_resurrect(key);
         PUSH(val);
-#endif
         CALL_SIMPLE_METHOD();
     }
 }
@@ -1365,9 +1363,7 @@ opt_aref_with
     val = vm_opt_aref_with(recv, key);
 
     if (val == Qundef) {
-#ifndef MJIT_HEADER
         PUSH(rb_str_resurrect(key));
-#endif
         CALL_SIMPLE_METHOD();
     }
 }
diff --git a/internal/bits.h b/internal/bits.h
index 2602ff7a31..6248e4cfa9 100644
--- a/internal/bits.h
+++ b/internal/bits.h
@@ -34,8 +34,7 @@
 # include <stdlib.h>            /* for _byteswap_uint64 */
 #endif
 
-#if defined(HAVE_X86INTRIN_H) && ! defined(MJIT_HEADER)
-# /* Rule out MJIT_HEADER, which does not interface well with <immintrin.h> */
+#if defined(HAVE_X86INTRIN_H)
 # include <x86intrin.h>         /* for _lzcnt_u64 */
 #elif MSC_VERSION_SINCE(1310)
 # include <intrin.h>            /* for the following intrinsics */
@@ -235,7 +234,7 @@ nlz_int32(uint32_t x)
      * safety. */
     return (unsigned int)__lzcnt(x);
 
-#elif defined(__x86_64__) && defined(__LZCNT__) && ! defined(MJIT_HEADER)
+#elif defined(__x86_64__) && defined(__LZCNT__)
     return (unsigned int)_lzcnt_u32(x);
 
 #elif MSC_VERSION_SINCE(1400) /* &&! defined(__AVX2__) */
@@ -264,7 +263,7 @@ nlz_int64(uint64_t x)
 #if defined(_MSC_VER) && defined(__AVX2__)
     return (unsigned int)__lzcnt64(x);
 
-#elif defined(__x86_64__) && defined(__LZCNT__) && ! defined(MJIT_HEADER)
+#elif defined(__x86_64__) && defined(__LZCNT__)
     return (unsigned int)_lzcnt_u64(x);
 
 #elif defined(_WIN64) && MSC_VERSION_SINCE(1400) /* &&! defined(__AVX2__) */
@@ -450,7 +449,7 @@ rb_popcount_intptr(uintptr_t x)
 static inline int
 ntz_int32(uint32_t x)
 {
-#if defined(__x86_64__) && defined(__BMI__) && ! defined(MJIT_HEADER)
+#if defined(__x86_64__) && defined(__BMI__)
     return (unsigned)_tzcnt_u32(x);
 
 #elif MSC_VERSION_SINCE(1400)
@@ -472,7 +471,7 @@ ntz_int32(uint32_t x)
 static inline int
 ntz_int64(uint64_t x)
 {
-#if defined(__x86_64__) && defined(__BMI__) && ! defined(MJIT_HEADER)
+#if defined(__x86_64__) && defined(__BMI__)
     return (unsigned)_tzcnt_u64(x);
 
 #elif defined(_WIN64) && MSC_VERSION_SINCE(1400)
diff --git a/internal/sanitizers.h b/internal/sanitizers.h
index 6e2d81137f..7b7d166c74 100644
--- a/internal/sanitizers.h
+++ b/internal/sanitizers.h
@@ -89,8 +89,6 @@
 # define VALGRIND_MAKE_MEM_UNDEFINED(p, n) 0
 #endif
 
-#ifndef MJIT_HEADER
-
 /*!
  * This function asserts that a (continuous) memory region from ptr to size
  * being "poisoned".  Both read / write access to such memory region are
@@ -185,6 +183,4 @@ asan_unpoison_object(VALUE obj, bool newobj_p)
     asan_unpoison_memory_region(ptr, SIZEOF_VALUE, newobj_p);
 }
 
-#endif /* MJIT_HEADER */
-
 #endif /* INTERNAL_SANITIZERS_H */
diff --git a/vm.c b/vm.c
index fe1411d174..c21d26cde9 100644
--- a/vm.c
+++ b/vm.c
@@ -43,11 +43,7 @@
 
 #include "builtin.h"
 
-#ifndef MJIT_HEADER
 #include "probes.h"
-#else
-#include "probes.dmyh"
-#endif
 #include "probes_helper.h"
 
 #ifdef RUBY_ASSERT_CRITICAL_SECTION
@@ -422,14 +418,10 @@ jit_exec(rb_execution_context_t *ec)
 
 #include "vm_insnhelper.c"
 
-#ifndef MJIT_HEADER
-
 #include "vm_exec.c"
 
 #include "vm_method.c"
-#endif /* #ifndef MJIT_HEADER */
 #include "vm_eval.c"
-#ifndef MJIT_HEADER
 
 #define PROCDEBUG 0
 
@@ -688,8 +680,6 @@ rb_vm_get_ruby_level_next_cfp(const rb_execution_context_t *ec, const rb_control
     return 0;
 }
 
-#endif /* #ifndef MJIT_HEADER */
-
 static rb_control_frame_t *
 vm_get_ruby_level_caller_cfp(const rb_execution_context_t *ec, const rb_control_frame_t *cfp)
 {
@@ -724,8 +714,6 @@ rb_vm_pop_cfunc_frame(void)
     vm_pop_frame(ec, cfp, cfp->ep);
 }
 
-#ifndef MJIT_HEADER
-
 void
 rb_vm_rewind_cfp(rb_execution_context_t *ec, rb_control_frame_t *cfp)
 {
@@ -4424,6 +4412,4 @@ rb_vm_empty_cc_for_super(void)
     return &vm_empty_cc_for_super;
 }
 
-#endif /* #ifndef MJIT_HEADER */
-
 #include "vm_call_iseq_optimized.inc" /* required from vm_insnhelper.c */
diff --git a/vm_eval.c b/vm_eval.c
index 7a9f5c4752..d8c48dd6e2 100644
--- a/vm_eval.c
+++ b/vm_eval.c
@@ -41,8 +41,6 @@ typedef enum call_type {
 static VALUE send_internal(int argc, const VALUE *argv, VALUE recv, call_type scope);
 static VALUE vm_call0_body(rb_execution_context_t* ec, struct rb_calling_info *calling, const VALUE *argv);
 
-#ifndef MJIT_HEADER
-
 VALUE
 rb_vm_call0(rb_execution_context_t *ec, VALUE recv, ID id, int argc, const VALUE *argv, const rb_callable_method_entry_t *cme, int kw_splat)
 {
@@ -352,8 +350,6 @@ rb_current_receiver(void)
     return cfp->self;
 }
 
-#endif /* #ifndef MJIT_HEADER */
-
 static inline void
 stack_check(rb_execution_context_t *ec)
 {
@@ -364,8 +360,6 @@ stack_check(rb_execution_context_t *ec)
     }
 }
 
-#ifndef MJIT_HEADER
-
 void
 rb_check_stack_overflow(void)
 {
@@ -944,8 +938,6 @@ rb_make_no_method_exception(VALUE exc, VALUE format, VALUE obj,
     }
 }
 
-#endif /* #ifndef MJIT_HEADER */
-
 static void
 raise_method_missing(rb_execution_context_t *ec, int argc, const VALUE *argv, VALUE obj,
                      enum method_missing_reason last_call_status)
@@ -1035,8 +1027,6 @@ method_missing(rb_execution_context_t *ec, VALUE obj, ID id, int argc, const VAL
     UNREACHABLE_RETURN(Qundef);
 }
 
-#ifndef MJIT_HEADER
-
 static inline VALUE
 rb_funcallv_scope(VALUE recv, ID mid, int argc, const VALUE *argv, call_type scope)
 {
@@ -2551,5 +2541,3 @@ Init_vm_eval(void)
     id_tag = rb_intern_const("tag");
     id_value = rb_intern_const("value");
 }
-
-#endif /* #ifndef MJIT_HEADER */
diff --git a/vm_exec.h b/vm_exec.h
index edea61d4f8..361516e86b 100644
--- a/vm_exec.h
+++ b/vm_exec.h
@@ -165,12 +165,6 @@ default:                        \
 
 #define VM_SP_CNT(ec, sp) ((sp) - (ec)->vm_stack)
 
-#ifdef MJIT_HEADER
-#define THROW_EXCEPTION(exc) do { \
-    ec->errinfo = (VALUE)(exc); \
-    EC_JUMP_TAG(ec, ec->tag->state); \
-} while (0)
-#else
 #if OPT_CALL_THREADED_CODE
 #define THROW_EXCEPTION(exc) do { \
     ec->errinfo = (VALUE)(exc); \
@@ -179,7 +173,6 @@ default:                        \
 #else
 #define THROW_EXCEPTION(exc) return (VALUE)(exc)
 #endif
-#endif
 
 #define SCREG(r) (reg_##r)
 
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 8a5d6e64a3..f1d50d179b 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -27,9 +27,7 @@
 
 /* finish iseq array */
 #include "insns.inc"
-#ifndef MJIT_HEADER
 #include "insns_info.inc"
-#endif
 
 extern rb_method_definition_t *rb_method_definition_create(rb_method_type_t type, ID mid);
 extern void rb_method_definition_set(const rb_method_entry_t *me, rb_method_definition_t *def, void *opts);
@@ -37,10 +35,8 @@ extern int rb_method_definition_eq(const rb_method_definition_t *d1, const rb_me
 extern VALUE rb_make_no_method_exception(VALUE exc, VALUE format, VALUE obj,
                                          int argc, const VALUE *argv, int priv);
 
-#ifndef MJIT_HEADER
 static const struct rb_callcache vm_empty_cc;
 static const struct rb_callcache vm_empty_cc_for_super;
-#endif
 
 /* control stack frame */
 
@@ -71,9 +67,6 @@ ec_stack_overflow(rb_execution_context_t *ec, int setup)
 }
 
 NORETURN(static void vm_stackoverflow(void));
-#ifdef MJIT_HEADER
-NOINLINE(static COLDFUNC void vm_stackoverflow(void));
-#endif
 
 static void
 vm_stackoverflow(void)
@@ -239,7 +232,6 @@ vm_check_frame(VALUE type,
 static VALUE vm_stack_canary; /* Initialized later */
 static bool vm_stack_canary_was_born = false;
 
-#ifndef MJIT_HEADER
 void
 rb_vm_check_canary(const rb_execution_context_t *ec, VALUE *sp)
 {
@@ -287,7 +279,6 @@ rb_vm_check_canary(const rb_execution_context_t *ec, VALUE *sp)
         name, stri, pos, strd);
     rb_bug("see above.");
 }
-#endif
 #define vm_check_canary(ec, sp) rb_vm_check_canary(ec, sp)
 
 #else
@@ -1981,8 +1972,6 @@ vm_ccs_verify(struct rb_class_cc_entries *ccs, ID mid, VALUE klass)
 }
 #endif
 
-#ifndef MJIT_HEADER
-
 static const rb_callable_method_entry_t *check_overloaded_cme(const rb_callable_method_entry_t *cme, const struct rb_callinfo * const ci);
 
 static const struct rb_callcache *
@@ -2103,7 +2092,6 @@ rb_vm_search_method_slowpath(const struct rb_callinfo *ci, VALUE klass)
 
     return cc;
 }
-#endif
 
 static const struct rb_callcache *
 vm_search_method_slowpath0(VALUE cd_owner, struct rb_call_data *cd, VALUE klass)
@@ -2117,12 +2105,7 @@ vm_search_method_slowpath0(VALUE cd_owner, struct rb_call_data *cd, VALUE klass)
 #if OPT_INLINE_METHOD_CACHE
     cd->cc = cc;
 
-    const struct rb_callcache *empty_cc =
-#ifdef MJIT_HEADER
-      rb_vm_empty_cc();
-#else
-      &vm_empty_cc;
-#endif
+    const struct rb_callcache *empty_cc = &vm_empty_cc;
     if (cd_owner && cc != empty_cc) RB_OBJ_WRITTEN(cd_owner, Qundef, cc);
 
 #if USE_DEBUG_COUNTER
@@ -2152,9 +2135,7 @@ vm_search_method_slowpath0(VALUE cd_owner, struct rb_call_data *cd, VALUE klass)
     return cc;
 }
 
-#ifndef MJIT_HEADER
 ALWAYS_INLINE(static const struct rb_callcache *vm_search_method_fastpath(VALUE cd_owner, struct rb_call_data *cd, VALUE klass));
-#endif
 static const struct rb_callcache *
 vm_search_method_fastpath(VALUE cd_owner, struct rb_call_data *cd, VALUE klass)
 {
@@ -2340,8 +2321,6 @@ opt_equality(const rb_iseq_t *cd_owner, VALUE recv, VALUE obj, CALL_DATA cd)
 
 #undef EQ_UNREDEFINED_P
 
-#ifndef MJIT_HEADER
-
 static inline const struct rb_callcache *gccct_method_search(rb_execution_context_t *ec, VALUE recv, ID mid, int argc); // vm_eval.c
 NOINLINE(static VALUE opt_equality_by_mid_slowpath(VALUE recv, VALUE obj, ID mid));
 
@@ -2382,8 +2361,6 @@ rb_eql_opt(VALUE obj1, VALUE obj2)
     return opt_equality_by_mid(obj1, obj2, idEqlP);
 }
 
-#endif // MJIT_HEADER
-
 extern VALUE rb_vm_call0(rb_execution_context_t *ec, VALUE, ID, int, const VALUE*, const rb_callable_method_entry_t *, int kw_splat);
 extern VALUE rb_vm_call_with_refinements(rb_execution_context_t *, VALUE, ID, int, const VALUE *, int);
 
@@ -4319,11 +4296,7 @@ vm_super_outside(void)
 static const struct rb_callcache *
 empty_cc_for_super(void)
 {
-#ifdef MJIT_HEADER
-    return rb_vm_empty_cc_for_super();
-#else
     return &vm_empty_cc_for_super;
-#endif
 }
 
 static const struct rb_callcache *
@@ -5195,52 +5168,19 @@ vm_invokeblock_i(struct rb_execution_context_struct *ec,
     }
 }
 
-#ifdef MJIT_HEADER
-static const struct rb_callcache *
-vm_search_method_wrap(const struct rb_control_frame_struct *reg_cfp, struct rb_call_data *cd, VALUE recv)
-{
-    return vm_search_method((VALUE)reg_cfp->iseq, cd, recv);
-}
-
-static const struct rb_callcache *
-vm_search_invokeblock(const struct rb_control_frame_struct *reg_cfp, struct rb_call_data *cd, VALUE recv)
-{
-    static const struct rb_callcache cc = {
-        .flags = T_IMEMO | (imemo_callcache << FL_USHIFT) | VM_CALLCACHE_UNMARKABLE,
-        .klass = 0,
-        .cme_  = 0,
-        .call_ = vm_invokeblock_i,
-        .aux_  = {0},
-    };
-    return &cc;
-}
-
-# define mexp_search_method vm_search_method_wrap
-# define mexp_search_super vm_search_super_method
-# define mexp_search_invokeblock vm_search_invokeblock
-#else
 enum method_explorer_type {
     mexp_search_method,
     mexp_search_invokeblock,
     mexp_search_super,
 };
-#endif
 
-static
-#ifndef MJIT_HEADER
-inline
-#endif
-VALUE
+static inline VALUE
 vm_sendish(
     struct rb_execution_context_struct *ec,
     struct rb_control_frame_struct *reg_cfp,
     struct rb_call_data *cd,
     VALUE block_handler,
-#ifdef MJIT_HEADER
-    const struct rb_callcache *(*method_explorer)(const struct rb_control_frame_struct *cfp, struct rb_call_data *cd, VALUE recv)
-#else
     enum method_explorer_type method_explorer
-#endif
 ) {
     VALUE val = Qundef;
     const struct rb_callinfo *ci = cd->ci;
@@ -5255,11 +5195,6 @@ vm_sendish(
         .ci = ci,
     };
 
-// The enum-based branch and inlining are faster in VM, but function pointers without inlining are faster in JIT.
-#ifdef MJIT_HEADER
-    calling.cc = cc = method_explorer(GET_CFP(), cd, recv);
-    val = vm_cc_call(cc)(ec, GET_CFP(), &calling);
-#else
     switch (method_explorer) {
       case mexp_search_method:
         calling.cc = cc = vm_search_method_fastpath((VALUE)reg_cfp->iseq, cd, CLASS_OF(recv));
@@ -5274,7 +5209,6 @@ vm_sendish(
         val = vm_invokeblock_i(ec, GET_CFP(), &calling);
         break;
     }
-#endif
 
     if (!UNDEF_P(val)) {
         return val;             /* CFUNC normal return */
@@ -5283,29 +5217,7 @@ vm_sendish(
         RESTORE_REGS();         /* CFP pushed in cc->call() */
     }
 
-#ifdef MJIT_HEADER
-    /* When calling ISeq which may catch an exception from JIT-ed
-       code, we should not call jit_exec directly to prevent the
-       caller frame from being canceled. That's because the caller
-       frame may have stack values in the local variables and the
-       cancelling the caller frame will purge them. But directly
-       calling jit_exec is faster... */
-    if (ISEQ_BODY(GET_ISEQ())->catch_except_p) {
-        VM_ENV_FLAGS_SET(GET_EP(), VM_FRAME_FLAG_FINISH);
-        return vm_exec(ec, true);
-    }
-    else if (UNDEF_P(val = jit_exec(ec))) {
-        VM_ENV_FLAGS_SET(GET_EP(), VM_FRAME_FLAG_FINISH);
-        return vm_exec(ec, false);
-    }
-    else {
-        return val;
-    }
-#else
-    /* When calling from VM, longjmp in the callee won't purge any
-       JIT-ed caller frames. So it's safe to directly call jit_exec. */
     return jit_exec(ec);
-#endif
 }
 
 /* object.c */
@@ -6298,7 +6210,6 @@ Init_vm_stack_canary(void)
     VM_ASSERT(n == 0);
 }
 
-#ifndef MJIT_HEADER
 void
 rb_vm_canary_is_found_dead(enum ruby_vminsn_type i, VALUE c)
 {
@@ -6310,7 +6221,6 @@ rb_vm_canary_is_found_dead(enum ruby_vminsn_type i, VALUE c)
 
     rb_bug("dead canary found at %s: %s", insn, str);
 }
-#endif
 
 #else
 void Init_vm_stack_canary(void) { /* nothing to do */ }
diff --git a/vm_insnhelper.h b/vm_insnhelper.h
index 3f0eaeb4be..db769f26e9 100644
--- a/vm_insnhelper.h
+++ b/vm_insnhelper.h
@@ -181,7 +181,6 @@ CC_SET_FASTPATH(const struct rb_callcache *cc, vm_call_handler func, bool enable
 /* others                                                 */
 /**********************************************************/
 
-#ifndef MJIT_HEADER
 #define CALL_SIMPLE_METHOD() do { \
     rb_snum_t x = leaf ? INSN_ATTR(width) : 0; \
     rb_snum_t y = attr_width_opt_send_without_block(0); \
@@ -189,7 +188,6 @@ CC_SET_FASTPATH(const struct rb_callcache *cc, vm_call_handler func, bool enable
     ADD_PC(z); \
     DISPATCH_ORIGINAL_INSN(opt_send_without_block); \
 } while (0)
-#endif
 
 #define GET_GLOBAL_CVAR_STATE() (ruby_vm_global_cvar_state)
 #define INC_GLOBAL_CVAR_STATE() (++ruby_vm_global_cvar_state)
diff --git a/yjit.h b/yjit.h
index 707c0f4b9d..765e735b9d 100644
--- a/yjit.h
+++ b/yjit.h
@@ -15,7 +15,7 @@
 # define YJIT_STATS RUBY_DEBUG
 #endif
 
-#if USE_YJIT && !defined(MJIT_HEADER) // MJIT and YJIT can't be enabled simultaneously
+#if USE_YJIT
 
 // We generate x86 or arm64 assembly
 #if defined(_WIN32) ? defined(_M_AMD64) : (defined(__x86_64__) || defined(__aarch64__))