diff --git a/flang/docs/OpenACC.md b/flang/docs/OpenACC.md
--- a/flang/docs/OpenACC.md
+++ b/flang/docs/OpenACC.md
@@ -15,3 +15,5 @@
 
 ## Intentional deviation from the specification
 * The end directive for combined construct can omit the `loop` keyword.
+* An `!$acc routine` with no parallelism clause is treated as if the `seq`
+  clause was present.
diff --git a/flang/test/Semantics/OpenACC/acc-routine-validity.f90 b/flang/test/Semantics/OpenACC/acc-routine-validity.f90
--- a/flang/test/Semantics/OpenACC/acc-routine-validity.f90
+++ b/flang/test/Semantics/OpenACC/acc-routine-validity.f90
@@ -10,7 +10,6 @@
 
   !$acc routine(fct2) vector
 
-  !ERROR: At least one of GANG, SEQ, VECTOR, WORKER clause must appear on the ROUTINE directive
   !$acc routine(sub3)
 
   !ERROR: ROUTINE directive without name must appear within the specification part of a subroutine or function definition, or within an interface body for a subroutine or function in an interface block
@@ -23,7 +22,6 @@
 
   subroutine sub1(a)
     real :: a(:)
-    !ERROR: At least one of GANG, SEQ, VECTOR, WORKER clause must appear on the ROUTINE directive
     !$acc routine
   end subroutine sub1
 
diff --git a/llvm/include/llvm/Frontend/OpenACC/ACC.td b/llvm/include/llvm/Frontend/OpenACC/ACC.td
--- a/llvm/include/llvm/Frontend/OpenACC/ACC.td
+++ b/llvm/include/llvm/Frontend/OpenACC/ACC.td
@@ -417,9 +417,7 @@
   let allowedOnceClauses = [
     VersionedClause<ACCC_Bind>,
     VersionedClause<ACCC_DeviceType>,
-    VersionedClause<ACCC_NoHost>
-  ];
-  let requiredClauses = [
+    VersionedClause<ACCC_NoHost>,
     VersionedClause<ACCC_Gang>,
     VersionedClause<ACCC_Seq>,
     VersionedClause<ACCC_Vector>,