diff --git a/clang/lib/Driver/SanitizerArgs.cpp b/clang/lib/Driver/SanitizerArgs.cpp
--- a/clang/lib/Driver/SanitizerArgs.cpp
+++ b/clang/lib/Driver/SanitizerArgs.cpp
@@ -19,6 +19,7 @@
 #include "llvm/Support/TargetParser.h"
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/TargetParser/AArch64TargetParser.h"
+#include "llvm/TargetParser/RISCVTargetParser.h"
 #include "llvm/Transforms/Instrumentation/AddressSanitizerOptions.h"
 #include <memory>
 
@@ -545,7 +546,8 @@
   if ((Kinds & SanitizerKind::ShadowCallStack) &&
       ((TC.getTriple().isAArch64() &&
         !llvm::AArch64::isX18ReservedByDefault(TC.getTriple())) ||
-       TC.getTriple().isRISCV()) &&
+       (TC.getTriple().isRISCV() &&
+        !llvm::RISCV::isX18ReservedByDefault(TC.getTriple()))) &&
       !Args.hasArg(options::OPT_ffixed_x18) && DiagnoseErrors) {
     D.Diag(diag::err_drv_argument_only_allowed_with)
         << lastArgumentForMask(D, Args, Kinds & SanitizerKind::ShadowCallStack)
diff --git a/clang/test/Driver/sanitizer-ld.c b/clang/test/Driver/sanitizer-ld.c
--- a/clang/test/Driver/sanitizer-ld.c
+++ b/clang/test/Driver/sanitizer-ld.c
@@ -731,6 +731,11 @@
 // RUN:   | FileCheck --check-prefix=CHECK-SHADOWCALLSTACK-LINUX-RISCV64 %s
 // CHECK-SHADOWCALLSTACK-LINUX-RISCV64: '-fsanitize=shadow-call-stack' only allowed with '-ffixed-x18'
 
+// RUN: %clang -fsanitize=shadow-call-stack -### %s 2>&1 \
+// RUN:     --target=riscv64-unknown-fuchsia -fuse-ld=ld \
+// RUN:   | FileCheck --check-prefix=CHECK-SHADOWCALLSTACK-FUCHSIA-RISCV64 %s
+// CHECK-SHADOWCALLSTACK-FUCHSIA-RISCV64-NOT: error:
+
 // RUN: %clang -fsanitize=shadow-call-stack -### %s 2>&1 \
 // RUN:     --target=aarch64-unknown-linux -fuse-ld=ld -ffixed-x18 \
 // RUN:   | FileCheck --check-prefix=CHECK-SHADOWCALLSTACK-LINUX-AARCH64-X18 %s
diff --git a/llvm/include/llvm/TargetParser/RISCVTargetParser.h b/llvm/include/llvm/TargetParser/RISCVTargetParser.h
--- a/llvm/include/llvm/TargetParser/RISCVTargetParser.h
+++ b/llvm/include/llvm/TargetParser/RISCVTargetParser.h
@@ -18,6 +18,9 @@
 #include <vector>
 
 namespace llvm {
+
+class Triple;
+
 namespace RISCV {
 
 // We use 64 bits as the known part in the scalable vector types.
@@ -38,6 +41,8 @@
 void fillValidTuneCPUArchList(SmallVectorImpl<StringRef> &Values, bool IsRV64);
 bool getCPUFeaturesExceptStdExt(CPUKind Kind, std::vector<StringRef> &Features);
 
+bool isX18ReservedByDefault(const Triple &TT);
+
 } // namespace RISCV
 } // namespace llvm
 
diff --git a/llvm/lib/Target/RISCV/RISCVSubtarget.cpp b/llvm/lib/Target/RISCV/RISCVSubtarget.cpp
--- a/llvm/lib/Target/RISCV/RISCVSubtarget.cpp
+++ b/llvm/lib/Target/RISCV/RISCVSubtarget.cpp
@@ -83,6 +83,9 @@
       FrameLowering(
           initializeSubtargetDependencies(TT, CPU, TuneCPU, FS, ABIName)),
       InstrInfo(*this), RegInfo(getHwMode()), TLInfo(TM, *this) {
+  if (RISCV::isX18ReservedByDefault(TT))
+    UserReservedRegister.set(RISCV::X18);
+
   CallLoweringInfo.reset(new RISCVCallLowering(*getTargetLowering()));
   Legalizer.reset(new RISCVLegalizerInfo(*this));
 
diff --git a/llvm/lib/TargetParser/RISCVTargetParser.cpp b/llvm/lib/TargetParser/RISCVTargetParser.cpp
--- a/llvm/lib/TargetParser/RISCVTargetParser.cpp
+++ b/llvm/lib/TargetParser/RISCVTargetParser.cpp
@@ -14,6 +14,7 @@
 #include "llvm/TargetParser/RISCVTargetParser.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringSwitch.h"
+#include "llvm/TargetParser/Triple.h"
 
 namespace llvm {
 namespace RISCV {
@@ -100,5 +101,10 @@
   return true;
 }
 
+bool isX18ReservedByDefault(const Triple &TT) {
+  // X18 is reserved for the ShadowCallStack ABI (even when not enabled).
+  return TT.isOSFuchsia();
+}
+
 } // namespace RISCV
 } // namespace llvm
diff --git a/llvm/test/CodeGen/RISCV/reserved-regs.ll b/llvm/test/CodeGen/RISCV/reserved-regs.ll
--- a/llvm/test/CodeGen/RISCV/reserved-regs.ll
+++ b/llvm/test/CodeGen/RISCV/reserved-regs.ll
@@ -57,6 +57,8 @@
 ; RUN: llc -mtriple=riscv32 -mattr=+reserve-x31 -verify-machineinstrs < %s | FileCheck %s -check-prefix=X31
 ; RUN: llc -mtriple=riscv64 -mattr=+reserve-x31 -verify-machineinstrs < %s | FileCheck %s -check-prefix=X31
 
+; RUN: llc -mtriple=riscv64-fuchsia -verify-machineinstrs < %s | FileCheck %s -check-prefix=X18
+
 ; This program is free to use all registers, but needs a stack pointer for
 ; spill values, so do not test for reserving the stack pointer.