diff --git a/lld/test/ELF/arm-ldrlit-err.s b/lld/test/ELF/arm-ldrlit-err.s
--- a/lld/test/ELF/arm-ldrlit-err.s
+++ b/lld/test/ELF/arm-ldrlit-err.s
@@ -8,6 +8,9 @@
   bx lr
   nop
   nop
+  nop
+  nop
+  nop
 
  .section .text.1, "ax", %progbits
  .global _start
@@ -27,3 +30,4 @@
  .balign 4
 high:
  bx lr
+ nop
diff --git a/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
--- a/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
+++ b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
@@ -11165,6 +11165,12 @@
                  "unexpected token in '.thumb_func' directive"))
     return true;
 
+  // .thumb_func implies .thumb
+  if (!isThumb())
+    SwitchMode();
+
+  getParser().getStreamer().emitAssemblerFlag(MCAF_Code16);
+
   NextSymbolIsThumb = true;
   return false;
 }
diff --git a/llvm/test/MC/ARM/thumb_func-implies-thumb.s b/llvm/test/MC/ARM/thumb_func-implies-thumb.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/ARM/thumb_func-implies-thumb.s
@@ -0,0 +1,31 @@
+@ RUN: llvm-mc -triple=armv7-darwin- -show-encoding < %s | FileCheck %s
+.syntax unified
+
+.text
+
+.arm
+@ Ensure the plain form switches mode.
+.thumb_func
+@ CHECK: .code 16
+@ CHECK-LABEL: foo
+foo:
+    bx lr
+
+.arm
+@ Ensure the labeled form doesn't switch mode.
+.thumb_func bar
+@ CHECK-NOT: .code 16
+@ CHECK-LABEL: bar
+bar:
+    bx lr
+
+.arm
+@ Ensure the nop is assembled in thumb mode, even though the baz symbol is
+@ defined later.
+.thumb_func
+nop
+@ CHECK: .code 16
+@ CHECK-NEXT: nop
+@ CHECK-LABEL: baz
+baz:
+    bx lr