diff --git a/llvm/include/llvm/MC/MCAsmInfo.h b/llvm/include/llvm/MC/MCAsmInfo.h
--- a/llvm/include/llvm/MC/MCAsmInfo.h
+++ b/llvm/include/llvm/MC/MCAsmInfo.h
@@ -202,6 +202,13 @@
   /// Defaults to false.
   bool AllowAtAtStartOfIdentifier = false;
 
+  /// This is true if the assembler allows the "#" character at the start of
+  /// a string to be lexed as an AsmToken::Identifier.
+  /// If the CommentString is also set to "#", setting this option will have
+  /// no effect, and the string will be lexed as a comment.
+  /// Defaults to false.
+  bool AllowHashAtStartOfIdentifier = false;
+
   /// If this is true, symbol names with invalid characters will be printed in
   /// quotes.
   bool SupportsQuotedNames = true;
@@ -626,6 +633,9 @@
   bool doesAllowDollarAtStartOfIdentifier() const {
     return AllowDollarAtStartOfIdentifier;
   }
+  bool doesAllowHashAtStartOfIdentifier() const {
+    return AllowHashAtStartOfIdentifier;
+  }
   bool supportsNameQuoting() const { return SupportsQuotedNames; }
 
   bool doesSupportDataRegionDirectives() const {
diff --git a/llvm/lib/MC/MCParser/AsmLexer.cpp b/llvm/lib/MC/MCParser/AsmLexer.cpp
--- a/llvm/lib/MC/MCParser/AsmLexer.cpp
+++ b/llvm/lib/MC/MCParser/AsmLexer.cpp
@@ -907,7 +907,11 @@
   case '/':
     IsAtStartOfStatement = OldIsAtStartOfStatement;
     return LexSlash();
-  case '#': return AsmToken(AsmToken::Hash, StringRef(TokStart, 1));
+  case '#': {
+    if (MAI.doesAllowHashAtStartOfIdentifier())
+      return LexIdentifier();
+    return AsmToken(AsmToken::Hash, StringRef(TokStart, 1));
+  }
   case '\'': return LexSingleQuote();
   case '"': return LexQuote();
   case '0': case '1': case '2': case '3': case '4':
diff --git a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmInfo.cpp b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmInfo.cpp
--- a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmInfo.cpp
+++ b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCAsmInfo.cpp
@@ -24,6 +24,10 @@
   CommentString = AssemblerDialect == AD_HLASM ? "*" : "#";
   RestrictCommentStringToStartOfStatement = (AssemblerDialect == AD_HLASM);
   AllowAdditionalComments = (AssemblerDialect == AD_ATT);
+  AllowAtAtStartOfIdentifier = (AssemblerDialect == AD_HLASM);
+  AllowDollarAtStartOfIdentifier = (AssemblerDialect == AD_HLASM);
+  AllowHashAtStartOfIdentifier = (AssemblerDialect == AD_HLASM);
+
   ZeroDirective = "\t.space\t";
   Data64bitsDirective = "\t.quad\t";
   UsesELFSectionDirectiveForBSS = true;
diff --git a/llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp b/llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp
--- a/llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp
+++ b/llvm/unittests/MC/SystemZ/SystemZAsmLexerTest.cpp
@@ -44,6 +44,9 @@
   void setAllowDollarAtStartOfIdentifier(bool Value) {
     AllowDollarAtStartOfIdentifier = Value;
   }
+  void setAllowHashAtStartOfIdentifier(bool Value) {
+    AllowHashAtStartOfIdentifier = Value;
+  }
 };
 
 // Setup a testing class that the GTest framework can call.
@@ -571,4 +574,22 @@
       {AsmToken::Identifier, AsmToken::EndOfStatement, AsmToken::Eof});
   lexAndCheckTokens(AsmStr, ExpectedTokens);
 }
+
+TEST_F(SystemZAsmLexerTest, CheckAcceptHashAtStartOfIdentifier) {
+  StringRef AsmStr = "##a#b$c";
+
+  // Setup.
+  MUPMAI->setAllowHashAtStartOfIdentifier(true);
+  MUPMAI->setCommentString("*");
+  MUPMAI->setAllowAdditionalComments(false);
+  setupCallToAsmParser(AsmStr);
+  Parser->getLexer().setAllowHashInIdentifier(true);
+
+  // Lex initially to get the string.
+  Parser->getLexer().Lex();
+
+  SmallVector<AsmToken::TokenKind> ExpectedTokens(
+      {AsmToken::Identifier, AsmToken::EndOfStatement, AsmToken::Eof});
+  lexAndCheckTokens(AsmStr, ExpectedTokens);
+}
 } // end anonymous namespace