diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -71,7 +71,9 @@
 Modified Compiler Flags
 -----------------------
 
-- ...
+- Support has been added for the following processors (``-mcpu`` identifiers in parentheses):
+
+  - RISC-V SiFive S51 (``sifive-s51``).
 
 Removed Compiler Flags
 -------------------------
diff --git a/clang/test/Driver/riscv-cpus.c b/clang/test/Driver/riscv-cpus.c
--- a/clang/test/Driver/riscv-cpus.c
+++ b/clang/test/Driver/riscv-cpus.c
@@ -45,6 +45,13 @@
 // RUN: %clang -target riscv64 -### -c %s 2>&1 -mtune=sifive-7-series | FileCheck -check-prefix=MTUNE-SIFIVE7-SERIES-64 %s
 // MTUNE-SIFIVE7-SERIES-64: "-tune-cpu" "sifive-7-rv64"
 
+// mcpu with mabi option
+// RUN: %clang -target riscv64 -### -c %s 2>&1 -mcpu=sifive-s51 -mabi=lp64 | FileCheck -check-prefix=MCPU-ABI-SIFIVE-S51 %s
+// MCPU-ABI-SIFIVE-S51: "-nostdsysteminc" "-target-cpu" "sifive-s51"
+// MCPU-ABI-SIFIVE-S51: "-target-feature" "+m" "-target-feature" "+a"
+// MCPU-ABI-SIFIVE-S51: "-target-feature" "+c" "-target-feature" "+64bit"
+// MCPU-ABI-SIFIVE-S51: "-target-abi" "lp64"
+
 // mcpu with default march
 // RUN: %clang -target riscv64 -### -c %s 2>&1 -mcpu=sifive-u54 | FileCheck -check-prefix=MCPU-SIFIVE-U54 %s
 // MCPU-SIFIVE-U54: "-nostdsysteminc" "-target-cpu" "sifive-u54"
diff --git a/clang/test/Misc/target-invalid-cpu-note.c b/clang/test/Misc/target-invalid-cpu-note.c
--- a/clang/test/Misc/target-invalid-cpu-note.c
+++ b/clang/test/Misc/target-invalid-cpu-note.c
@@ -196,7 +196,7 @@
 
 // RUN: not %clang_cc1 -triple riscv64 -target-cpu not-a-cpu -fsyntax-only %s 2>&1 | FileCheck %s --check-prefix RISCV64
 // RISCV64: error: unknown target CPU 'not-a-cpu'
-// RISCV64: note: valid target CPU values are: generic-rv64, rocket-rv64, sifive-7-rv64, sifive-u54, sifive-u74
+// RISCV64: note: valid target CPU values are: generic-rv64, rocket-rv64, sifive-7-rv64, sifive-s51, sifive-u54, sifive-u74
 
 // RUN: not %clang_cc1 -triple riscv32 -tune-cpu not-a-cpu -fsyntax-only %s 2>&1 | FileCheck %s --check-prefix TUNE-RISCV32
 // TUNE-RISCV32: error: unknown target CPU 'not-a-cpu'
@@ -204,4 +204,4 @@
 
 // RUN: not %clang_cc1 -triple riscv64 -tune-cpu not-a-cpu -fsyntax-only %s 2>&1 | FileCheck %s --check-prefix TUNE-RISCV64
 // TUNE-RISCV64: error: unknown target CPU 'not-a-cpu'
-// TUNE-RISCV64: note: valid target CPU values are: generic-rv64, rocket-rv64, sifive-7-rv64, sifive-u54, sifive-u74, generic, rocket, sifive-7-series
+// TUNE-RISCV64: note: valid target CPU values are: generic-rv64, rocket-rv64, sifive-7-rv64, sifive-s51, sifive-u54, sifive-u74, generic, rocket, sifive-7-series
diff --git a/llvm/include/llvm/Support/RISCVTargetParser.def b/llvm/include/llvm/Support/RISCVTargetParser.def
--- a/llvm/include/llvm/Support/RISCVTargetParser.def
+++ b/llvm/include/llvm/Support/RISCVTargetParser.def
@@ -20,6 +20,7 @@
 PROC(SIFIVE_732, {"sifive-7-rv32"}, FK_NONE, {""})
 PROC(SIFIVE_764, {"sifive-7-rv64"}, FK_64BIT, {""})
 PROC(SIFIVE_E31, {"sifive-e31"}, FK_NONE, {"rv32imac"})
+PROC(SIFIVE_S51, {"sifive-s51"}, FK_64BIT, {"rv64imac"})
 PROC(SIFIVE_U54, {"sifive-u54"}, FK_64BIT, {"rv64gc"})
 PROC(SIFIVE_E76, {"sifive-e76"}, FK_NONE, {"rv32imafc"})
 PROC(SIFIVE_U74, {"sifive-u74"}, FK_64BIT, {"rv64gc"})
diff --git a/llvm/lib/Target/RISCV/RISCV.td b/llvm/lib/Target/RISCV/RISCV.td
--- a/llvm/lib/Target/RISCV/RISCV.td
+++ b/llvm/lib/Target/RISCV/RISCV.td
@@ -254,6 +254,11 @@
                                                  FeatureStdExtA,
                                                  FeatureStdExtC]>;
 
+def : ProcessorModel<"sifive-s51", RocketModel, [Feature64Bit,
+                                                 FeatureStdExtM,
+                                                 FeatureStdExtA,
+                                                 FeatureStdExtC]>;
+
 def : ProcessorModel<"sifive-u54", RocketModel, [Feature64Bit,
                                                  FeatureStdExtM,
                                                  FeatureStdExtA,