diff --git a/toolkit/components/telemetry/core/TelemetryOrigin.cpp b/toolkit/components/telemetry/core/TelemetryOrigin.cpp
index cb3e3810dbff..fa35ab594ef3 100644
--- a/toolkit/components/telemetry/core/TelemetryOrigin.cpp
+++ b/toolkit/components/telemetry/core/TelemetryOrigin.cpp
@@ -135,8 +135,10 @@ static uint32_t gPrioDataCount = 0;
 static uint32_t gPrioDatasPerMetric;
 
 // The number of "meta-origins": in-band metadata about origin telemetry.
-// Currently 0. One is planned: the "unknown origin recorded" meta-origin.
-static uint32_t kNumMetaOrigins = 0;
+// Currently 1: the "unknown origin recorded" meta-origin.
+static uint32_t kNumMetaOrigins = 1;
+
+NS_NAMED_LITERAL_CSTRING(kUnknownOrigin, "__UNKNOWN__");
 
 }  // namespace
 
@@ -222,9 +224,14 @@ void TelemetryOrigin::InitializeGlobalState() {
 
   gOriginToIndexMap = new OriginToIndexMap(gOriginsList->Length());
   for (size_t i = 0; i < gOriginsList->Length(); ++i) {
+    MOZ_ASSERT(!kUnknownOrigin.Equals((*gOriginsList)[i]),
+               "Unknown origin literal is reserved in Origin Telemetry");
     gOriginToIndexMap->Put(nsDependentCString((*gOriginsList)[i]), i);
   }
 
+  // Add the meta-origin for tracking recordings to untracked origins.
+  gOriginToIndexMap->Put(kUnknownOrigin, gOriginsList->Length());
+
   gMetricToOriginsMap = new IdToOriginsMap();
 
   // This map shouldn't change at runtime, so make debug builds complain
@@ -274,8 +281,15 @@ nsresult TelemetryOrigin::RecordOrigin(OriginMetricID aId,
     return NS_OK;
   }
 
+  nsCString origin(aOrigin);
   if (!gOriginToIndexMap->Contains(aOrigin)) {
-    return NS_OK;
+    // Only record one unknown origin per metric per snapshot.
+    // (otherwise we may get swamped and blow our data budget.)
+    if (gMetricToOriginsMap->Contains(aId) &&
+        gMetricToOriginsMap->GetOrInsert(aId).Contains(kUnknownOrigin)) {
+      return NS_OK;
+    }
+    origin = kUnknownOrigin;
   }
 
   if (!gMetricToOriginsMap->Contains(aId)) {
@@ -286,13 +300,13 @@ nsresult TelemetryOrigin::RecordOrigin(OriginMetricID aId,
 
   auto& originArray = gMetricToOriginsMap->GetOrInsert(aId);
 
-  if (originArray.Contains(aOrigin)) {
+  if (originArray.Contains(origin)) {
     // If we've already recorded this metric for this origin, then we're going
     // to need more prioDatas to encode that it happened again.
     gPrioDataCount += gPrioDatasPerMetric;
   }
 
-  originArray.AppendElement(aOrigin);
+  originArray.AppendElement(origin);
 
   static uint32_t sPrioPingLimit =
       mozilla::Preferences::GetUint("toolkit.telemetry.prioping.dataLimit", 10);