diff --git a/clang/lib/Lex/LiteralSupport.cpp b/clang/lib/Lex/LiteralSupport.cpp
index 75ad977d64b24..3b941fc4c48b8 100644
--- a/clang/lib/Lex/LiteralSupport.cpp
+++ b/clang/lib/Lex/LiteralSupport.cpp
@@ -1420,7 +1420,7 @@ void NumericLiteralParser::ParseNumberStartingWithZero(SourceLocation TokLoc) {
   }
 
   // Parse a potential octal literal prefix.
-  bool SawOctalPrefix = false, IsSingleZero = false;
+  bool IsSingleZero = false;
   if ((c1 == 'O' || c1 == 'o') && (s[1] >= '0' && s[1] <= '7')) {
     unsigned DiagId;
     if (LangOpts.C2y)
@@ -1432,14 +1432,26 @@ void NumericLiteralParser::ParseNumberStartingWithZero(SourceLocation TokLoc) {
     Diags.Report(TokLoc, DiagId);
     ++s;
     DigitsBegin = s;
-    SawOctalPrefix = true;
+    radix = 8;
+    s = SkipOctalDigits(s);
+    if (s == ThisTokEnd) {
+      // Done
+    } else if ((isHexDigit(*s) && *s != 'e' && *s != 'E' && *s != '.') &&
+               !isValidUDSuffix(LangOpts, StringRef(s, ThisTokEnd - s))) {
+      Diags.Report(Lexer::AdvanceToTokenCharacter(TokLoc, s - ThisTokBegin, SM,
+                                                  LangOpts),
+                   diag::err_invalid_digit)
+          << StringRef(s, 1) << 1;
+      hadError = true;
+    }
+    // Other suffixes will be diagnosed by the caller.
+    return;
   }
 
   auto _ = llvm::make_scope_exit([&] {
     // If we still have an octal value but we did not see an octal prefix,
     // diagnose as being an obsolescent feature starting in C2y.
-    if (radix == 8 && LangOpts.C2y && !SawOctalPrefix && !hadError &&
-        !IsSingleZero)
+    if (radix == 8 && LangOpts.C2y && !hadError && !IsSingleZero)
       Diags.Report(TokLoc, diag::warn_unprefixed_octal_deprecated);
   });
 
diff --git a/clang/test/C/C2y/n3353.c b/clang/test/C/C2y/n3353.c
index a616228f1bad0..d7d8b03501260 100644
--- a/clang/test/C/C2y/n3353.c
+++ b/clang/test/C/C2y/n3353.c
@@ -92,6 +92,28 @@ int o2 = 0xG; /* expected-error {{invalid suffix 'xG' on integer constant}}
                  c2y-warning {{octal literals without a '0o' prefix are deprecated}}
                */
 
+// Show that floating-point suffixes on octal literals are rejected.
+auto f1 = 0o0.;  /* expected-error {{invalid suffix '.' on integer constant}}
+                    compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                    ext-warning {{octal integer literals are a C2y extension}}
+                    cpp-warning {{octal integer literals are a Clang extension}}
+                */
+auto f2 = 0o0.1; /* expected-error {{invalid suffix '.1' on integer constant}}
+                    compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                    ext-warning {{octal integer literals are a C2y extension}}
+                    cpp-warning {{octal integer literals are a Clang extension}}
+                */
+auto f3 = 0o0e1; /* expected-error {{invalid suffix 'e1' on integer constant}}
+                    compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                    ext-warning {{octal integer literals are a C2y extension}}
+                    cpp-warning {{octal integer literals are a Clang extension}}
+                 */
+auto f4 = 0o0E1; /* expected-error {{invalid suffix 'E1' on integer constant}}
+                    compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                    ext-warning {{octal integer literals are a C2y extension}}
+                    cpp-warning {{octal integer literals are a Clang extension}}
+                 */
+
 // Ensure digit separators work as expected.
 constexpr int p = 0o0'1'2'3'4'5'6'7; /* compat-warning {{octal integer literals are incompatible with standards before C2y}}
                                         ext-warning {{octal integer literals are a C2y extension}}