diff --git a/NEWS.md b/NEWS.md
index 1ce8995e4b..383e6ed04d 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -12,6 +12,7 @@ Note that each entry is kept to a minimum, see links for details.
 * A new `performance` warning category was introduced.
   They are not displayed by default even in verbose mode.
   Turn them on with `-W:performance` or `Warning[:performance] = true`. [[Feature #19538]]
+* The `RUBY_GC_HEAP_INIT_SLOTS` environment variable has been deprecated and removed. Environment variables `RUBY_GC_HEAP_INIT_SIZE_%d_SLOTS` should be used instead. [[Feature #19785]]
 
 ## Core classes updates
 
@@ -164,3 +165,4 @@ changelog for details of the default gems or bundled gems.
 [Feature #19538]: https://bugs.ruby-lang.org/issues/19538
 [Feature #19591]: https://bugs.ruby-lang.org/issues/19591
 [Feature #19714]: https://bugs.ruby-lang.org/issues/19714
+[Feature #19785]: https://bugs.ruby-lang.org/issues/19785
diff --git a/gc.c b/gc.c
index 758f65c2c8..6c4bffa95d 100644
--- a/gc.c
+++ b/gc.c
@@ -11615,8 +11615,6 @@ gc_set_initial_pages(rb_objspace_t *objspace)
 /*
  * GC tuning environment variables
  *
- * * RUBY_GC_HEAP_INIT_SLOTS
- *   - Initial allocation slots.
  * * RUBY_GC_HEAP_FREE_SLOTS
  *   - Prepare at least this amount of slots after GC.
  *   - Allocate slots if there are not enough slots.
@@ -11663,13 +11661,6 @@ ruby_gc_set_params(void)
         /* ok */
     }
 
-    /* RUBY_GC_HEAP_INIT_SLOTS */
-    size_t global_init_slots = GC_HEAP_INIT_SLOTS;
-    if (get_envparam_size("RUBY_GC_HEAP_INIT_SLOTS", &global_init_slots, 0)) {
-        for (int i = 0; i < SIZE_POOL_COUNT; i++) {
-            gc_params.size_pool_init_slots[i] = global_init_slots;
-        }
-    }
     gc_set_initial_pages(objspace);
 
     get_envparam_double("RUBY_GC_HEAP_GROWTH_FACTOR", &gc_params.growth_factor, 1.0, 0.0, FALSE);
diff --git a/ruby.c b/ruby.c
index 2b4397c001..213425950f 100644
--- a/ruby.c
+++ b/ruby.c
@@ -1738,6 +1738,13 @@ ruby_opt_init(ruby_cmdline_options_t *opt)
 
     rb_warning_category_update(opt->warn.mask, opt->warn.set);
 
+    /* [Feature #19785] Warning for removed GC environment variable.
+     * Remove this in Ruby 3.4. */
+    if (getenv("RUBY_GC_HEAP_INIT_SLOTS")) {
+        rb_warn_deprecated("The environment variable RUBY_GC_HEAP_INIT_SLOTS",
+                           "environment variables RUBY_GC_HEAP_INIT_SIZE_%d_SLOTS");
+    }
+
 #if USE_RJIT
     // rb_call_builtin_inits depends on RubyVM::RJIT.enabled?
     if (opt->rjit.on)
diff --git a/test/ruby/test_gc.rb b/test/ruby/test_gc.rb
index bfcfbbb72f..64a860fe65 100644
--- a/test/ruby/test_gc.rb
+++ b/test/ruby/test_gc.rb
@@ -375,13 +375,9 @@ class TestGc < Test::Unit::TestCase
     env = {
       "RUBY_GC_HEAP_INIT_SLOTS" => "100"
     }
-    assert_in_out_err([env, "-W0", "-e", "exit"], "", [], [], "[Bug #19284]")
-
-    env = {
-      "RUBY_GC_MALLOC_LIMIT" => "60000000",
-      "RUBY_GC_HEAP_INIT_SLOTS" => "100000"
-    }
-    assert_normal_exit("exit", "[ruby-core:39777]", :child_env => env)
+    assert_in_out_err([env, "-W0", "-e", "exit"], "", [], [])
+    assert_in_out_err([env, "-W:deprecated", "-e", "exit"], "", [],
+                       /The environment variable RUBY_GC_HEAP_INIT_SLOTS is deprecated; use environment variables RUBY_GC_HEAP_INIT_SIZE_%d_SLOTS instead/)
 
     env = {}
     GC.stat_heap.each do |_, s|
@@ -389,26 +385,12 @@ class TestGc < Test::Unit::TestCase
     end
     assert_normal_exit("exit", "", :child_env => env)
 
-    env["RUBY_GC_HEAP_INIT_SLOTS"] = "100000"
-    assert_normal_exit("exit", "", :child_env => env)
-
     env = {}
     GC.stat_heap.each do |_, s|
       env["RUBY_GC_HEAP_INIT_SIZE_#{s[:slot_size]}_SLOTS"] = "0"
     end
     assert_normal_exit("exit", "", :child_env => env)
 
-    env = {
-      "RUBYOPT" => "",
-      "RUBY_GC_HEAP_INIT_SLOTS" => "100000"
-    }
-    assert_in_out_err([env, "-e", "exit"], "", [], [], "[ruby-core:39795]")
-    assert_in_out_err([env, "-W0", "-e", "exit"], "", [], [], "[ruby-core:39795]")
-    assert_in_out_err([env, "-W1", "-e", "exit"], "", [], [], "[ruby-core:39795]")
-    assert_in_out_err([env, "-w", "-e", "exit"], "", [], /RUBY_GC_HEAP_INIT_SLOTS=100000/, "[ruby-core:39795]")
-    # Value of GC_HEAP_INIT_SLOTS is 10000
-    assert_in_out_err([env, "-w", "-e", "exit"], "", [], /\(default value: 10000\)/)
-
     env = {
       "RUBY_GC_HEAP_GROWTH_FACTOR" => "2.0",
       "RUBY_GC_HEAP_GROWTH_MAX_SLOTS" => "10000"
@@ -417,15 +399,10 @@ class TestGc < Test::Unit::TestCase
     assert_in_out_err([env, "-w", "-e", "exit"], "", [], /RUBY_GC_HEAP_GROWTH_FACTOR=2.0/, "")
     assert_in_out_err([env, "-w", "-e", "exit"], "", [], /RUBY_GC_HEAP_GROWTH_MAX_SLOTS=10000/, "[ruby-core:57928]")
 
-    env = {
-      "RUBY_GC_HEAP_INIT_SLOTS" => "100000",
-      "RUBY_GC_HEAP_FREE_SLOTS" => "10000",
-      "RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR" => "0.4",
-    }
-    assert_normal_exit("exit", "", :child_env => env)
-    assert_in_out_err([env, "-w", "-e", "exit"], "", [], /RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR=0\.4/, "")
-
     if use_rgengc?
+      env = {
+        "RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR" => "0.4",
+      }
       # always full GC when RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR < 1.0
       assert_in_out_err([env, "--disable-gems", "-e", "GC.start; 1000_000.times{Object.new}; p(GC.stat[:minor_gc_count] < GC.stat[:major_gc_count])"], "", ['true'], //, "")
     end