diff --git a/include/swift/AST/DiagnosticGroups.def b/include/swift/AST/DiagnosticGroups.def
index f5eab826de0f8..dd1e8a8cd3390 100644
--- a/include/swift/AST/DiagnosticGroups.def
+++ b/include/swift/AST/DiagnosticGroups.def
@@ -41,6 +41,7 @@
 GROUP(no_group, "")
 
 GROUP(ActorIsolatedCall, "actor-isolated-call")
+GROUP(AvailabilityUnrecognizedName, "availability-unrecognized-name")
 GROUP(ClangDeclarationImport, "clang-declaration-import")
 GROUP(ConformanceIsolation, "conformance-isolation")
 GROUP(DeprecatedDeclaration, "deprecated-declaration")
diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index f79d13ab8af2a..3541aed714058 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -6913,11 +6913,12 @@ ERROR(availability_must_occur_alone, none,
       (AvailabilityDomain, bool))
 ERROR(availability_unexpected_version, none,
       "unexpected version number for %0", (AvailabilityDomain))
-WARNING(availability_unrecognized_platform_name,
-        PointsToFirstBadToken, "unrecognized platform name %0", (Identifier))
-WARNING(availability_suggest_platform_name,
-        PointsToFirstBadToken, "unrecognized platform name %0;"
-        " did you mean '%1'?",
+GROUPED_ERROR(availability_unrecognized_platform_name,
+        AvailabilityUnrecognizedName, PointsToFirstBadToken,
+        "unrecognized platform name %0", (Identifier))
+GROUPED_ERROR(availability_suggest_platform_name,
+        AvailabilityUnrecognizedName, PointsToFirstBadToken,
+        "unrecognized platform name %0; did you mean '%1'?",
         (Identifier, StringRef))
 WARNING(availability_unsupported_version_number, none,
         "'%0' is not a supported version number", (llvm::VersionTuple))
diff --git a/lib/AST/AvailabilityDomain.cpp b/lib/AST/AvailabilityDomain.cpp
index 96e5d48d07cdd..bc2e0a2fa9272 100644
--- a/lib/AST/AvailabilityDomain.cpp
+++ b/lib/AST/AvailabilityDomain.cpp
@@ -368,22 +368,29 @@ AvailabilityDomainOrIdentifier::lookUpInDeclContext(
     domain = results.front();
   }
 
+  bool hasCustomAvailability =
+      ctx.LangOpts.hasFeature(Feature::CustomAvailability);
+
   if (!domain) {
     auto domainString = identifier.str();
+    bool downgradeErrors =
+        !hasCustomAvailability || declContext->isInSwiftinterface();
     if (auto suggestion = closestCorrectedPlatformString(domainString)) {
       diags
           .diagnose(loc, diag::availability_suggest_platform_name, identifier,
                     *suggestion)
+          .limitBehaviorIf(downgradeErrors, DiagnosticBehavior::Warning)
           .fixItReplace(SourceRange(loc), *suggestion);
     } else {
-      diags.diagnose(loc, diag::availability_unrecognized_platform_name,
-                     identifier);
+      diags
+          .diagnose(loc, diag::availability_unrecognized_platform_name,
+                    identifier)
+          .limitBehaviorIf(downgradeErrors, DiagnosticBehavior::Warning);
     }
     return std::nullopt;
   }
 
-  if (domain->isCustom() &&
-      !ctx.LangOpts.hasFeature(Feature::CustomAvailability) &&
+  if (domain->isCustom() && !hasCustomAvailability &&
       !declContext->isInSwiftinterface()) {
     diags.diagnose(loc, diag::attr_availability_requires_custom_availability,
                    *domain);
diff --git a/test/ClangImporter/Inputs/availability_custom_domains_other.swift b/test/ClangImporter/Inputs/availability_custom_domains_other.swift
index f51a436531000..43477f09c8185 100644
--- a/test/ClangImporter/Inputs/availability_custom_domains_other.swift
+++ b/test/ClangImporter/Inputs/availability_custom_domains_other.swift
@@ -1,6 +1,6 @@
 import Seas
 
-@available(Arctic) // expected-warning {{unrecognized platform name 'Arctic'}}
+@available(Arctic) // expected-error {{unrecognized platform name 'Arctic'}}
 func availableInArctic() { }
 
 @available(Mediterranean)
diff --git a/test/ClangImporter/availability_custom_domains.swift b/test/ClangImporter/availability_custom_domains.swift
index 0a87fae61bddb..ca81219d69645 100644
--- a/test/ClangImporter/availability_custom_domains.swift
+++ b/test/ClangImporter/availability_custom_domains.swift
@@ -44,7 +44,7 @@ func availableInPacific() { }
 func unavailableInColorado() { } // expected-note {{'unavailableInColorado()' has been explicitly marked unavailable here}}
 
 // The Seas module is only imported directly by the other source file.
-@available(Baltic) // expected-warning {{unrecognized platform name 'Baltic'}}
+@available(Baltic) // expected-error {{unrecognized platform name 'Baltic'}}
 func availableInBaltic() { } // expected-note {{did you mean 'availableInBaltic'}}
 
 func testSwiftDecls() {
diff --git a/test/Parse/availability_query_custom_domains.swift b/test/Parse/availability_query_custom_domains.swift
index 1b176b437bc42..e556a17151bc9 100644
--- a/test/Parse/availability_query_custom_domains.swift
+++ b/test/Parse/availability_query_custom_domains.swift
@@ -9,13 +9,13 @@
 if #available(EnabledDomain) { }
 if #available(DisabledDomain) { }
 if #available(DynamicDomain) { }
-if #available(UnknownDomain) { } // expected-warning {{unrecognized platform name 'UnknownDomain'}}
+if #available(UnknownDomain) { } // expected-error {{unrecognized platform name 'UnknownDomain'}}
 // expected-error@-1 {{condition required for target platform}}
 
 if #unavailable(EnabledDomain) { }
 if #unavailable(DisabledDomain) { }
 if #unavailable(DynamicDomain) { }
-if #unavailable(UnknownDomain) { } // expected-warning {{unrecognized platform name 'UnknownDomain'}}
+if #unavailable(UnknownDomain) { } // expected-error {{unrecognized platform name 'UnknownDomain'}}
 
 if #available(EnabledDomain 1.0) { } // expected-error {{unexpected version number for EnabledDomain}}
 if #available(EnabledDomain, DisabledDomain) { } // expected-error {{EnabledDomain availability must be specified alone}}
diff --git a/test/Sema/availability_custom_domains.swift b/test/Sema/availability_custom_domains.swift
index 4caac2ec87824..39a18d0408eab 100644
--- a/test/Sema/availability_custom_domains.swift
+++ b/test/Sema/availability_custom_domains.swift
@@ -26,7 +26,7 @@ func deprecatedInDynamicDomain() { }
 @available(DynamicDomain, unavailable)
 func unavailableInDynamicDomain() { } // expected-note * {{'unavailableInDynamicDomain()' has been explicitly marked unavailable here}}
 
-@available(UnknownDomain) // expected-warning {{unrecognized platform name 'UnknownDomain'}}
+@available(UnknownDomain) // expected-error {{unrecognized platform name 'UnknownDomain'}}
 func availableInUnknownDomain() { }
 
 func testDeployment() {
diff --git a/test/attr/attr_availability_custom_domains.swift b/test/attr/attr_availability_custom_domains.swift
index 4dba8063eec3e..14d0f3f62d897 100644
--- a/test/attr/attr_availability_custom_domains.swift
+++ b/test/attr/attr_availability_custom_domains.swift
@@ -57,5 +57,5 @@ func deprecatedInRedefinedDomain() { }
 @available(DynamicDomain)
 func availableInDynamicDomain() { }
 
-@available(UnknownDomain) // expected-warning {{unrecognized platform name 'UnknownDomain'}}
+@available(UnknownDomain) // expected-error {{unrecognized platform name 'UnknownDomain'}}
 func availableInUnknownDomain() { }
diff --git a/userdocs/diagnostics/availability-unrecognized-name.md b/userdocs/diagnostics/availability-unrecognized-name.md
new file mode 100644
index 0000000000000..1caf5bb107c11
--- /dev/null
+++ b/userdocs/diagnostics/availability-unrecognized-name.md
@@ -0,0 +1,18 @@
+# Unrecognized availability platforms (AvailabilityUnrecognizedName)
+
+Warnings that identify unrecognized platform names in `@available` attributes and `if #available` statements.
+
+## Overview
+
+The `AvailabilityUnrecognizedName` group covers warnings emitted when the platform name specified in an availability related construct is unrecognized by the compiler:
+
+```
+@available(NotAValidPlatform, introduced: 1.0) // warning: unrecognized platform name 'NotAValidPlatform'
+public func function() {
+  if #available(NotAValidPlatform 2.0, *) { // warning: unrecognized platform name 'NotAValidPlatform'
+    // ...
+  }
+}
+```
+
+Availability specifications with unrecognized platform names in `@available` attributes and `#available` queries are ignored by the compiler.