diff --git a/AUTHORS b/AUTHORS
index 4a14b98707..a414be01ee 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -79,6 +79,7 @@ Stanley C. <stanbot8@gmail.com>
 Steinar H. Gunderson <sgunderson@bigfoot.com>
 Stripe, Inc.
 Tobias Schmidt <tobias.schmidt@in.tum.de>
+Vincenzo Reina <vincenzoreinaa05@gmail.com>
 XiaoWei Lu <shaway77606@gmail.com>
 Yixuan Qiu <yixuanq@gmail.com>
 Yusuke Suzuki <utatane.tea@gmail.com>
diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index 74e5fe958f..ea6c3a2bb4 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -106,6 +106,7 @@ Stanley C. <stanbot8@gmail.com>
 Tobias Schmidt <tobias.schmidt@in.tum.de>
 Tobias Ulvgård <tobias.ulvgard@dirac.se>
 Tom Madams <tom.ej.madams@gmail.com> <tmadams@google.com>
+Vincenzo Reina <vincenzoreinaa05@gmail.com>
 XiaoWei Lu <shaway77606@gmail.com>
 Yixuan Qiu <yixuanq@gmail.com>
 Yusuke Suzuki <utatane.tea@gmail.com>
diff --git a/include/benchmark/sysinfo.h b/include/benchmark/sysinfo.h
index 711e64d7fb..08ca4fbbba 100644
--- a/include/benchmark/sysinfo.h
+++ b/include/benchmark/sysinfo.h
@@ -38,6 +38,7 @@ struct BENCHMARK_EXPORT CPUInfo {
   enum Scaling { UNKNOWN, ENABLED, DISABLED };
 
   int num_cpus;
+  int num_cpus_in_affinity_mask;
   Scaling scaling;
   double cycles_per_second;
   std::vector<CacheInfo> caches;
diff --git a/src/json_reporter.cc b/src/json_reporter.cc
index 37da17ba03..740a0992e2 100644
--- a/src/json_reporter.cc
+++ b/src/json_reporter.cc
@@ -139,6 +139,12 @@ bool JSONReporter::ReportContext(const Context& context) {
   CPUInfo const& info = context.cpu_info;
   out << indent << FormatKV("num_cpus", static_cast<int64_t>(info.num_cpus))
       << ",\n";
+  if (info.num_cpus_in_affinity_mask > 0) {
+    out << indent
+        << FormatKV("num_cpus_in_affinity_mask",
+                    static_cast<int64_t>(info.num_cpus_in_affinity_mask))
+        << ",\n";
+  }
   out << indent
       << FormatKV("mhz_per_cpu",
                   RoundDouble(info.cycles_per_second / 1000000.0))
diff --git a/src/reporter.cc b/src/reporter.cc
index 298ff1a60e..91649d9dfa 100644
--- a/src/reporter.cc
+++ b/src/reporter.cc
@@ -54,7 +54,13 @@ void BenchmarkReporter::PrintBasicContext(std::ostream* out,
   const CPUInfo& info = context.cpu_info;
   Out << "Run on (" << info.num_cpus << " X "
       << (info.cycles_per_second / 1000000.0) << " MHz CPU "
-      << ((info.num_cpus > 1) ? "s" : "") << ")\n";
+      << ((info.num_cpus > 1) ? "s" : "");
+  // Only mention the affinity mask when it actually restricts the process.
+  if (info.num_cpus_in_affinity_mask > 0 &&
+      info.num_cpus_in_affinity_mask < info.num_cpus) {
+    Out << ", " << info.num_cpus_in_affinity_mask << " in affinity mask";
+  }
+  Out << ")\n";
   if (!info.caches.empty()) {
     Out << "CPU Caches:\n";
     for (const auto& CInfo : info.caches) {
diff --git a/src/sysinfo.cc b/src/sysinfo.cc
index a291e10e2b..a1485717af 100644
--- a/src/sysinfo.cc
+++ b/src/sysinfo.cc
@@ -574,6 +574,34 @@ int GetNumCPUs() {
   return num_cpus;
 }
 
+int GetNumCPUsInAffinityMask() {
+#if defined(BENCHMARK_HAS_PTHREAD_AFFINITY)
+  cpu_set_t affinity;
+  if (pthread_getaffinity_np(pthread_self(), sizeof(affinity), &affinity) !=
+      0) {
+    return 0;
+  }
+  int num_cpus = 0;
+  for (int i = 0; i < CPU_SETSIZE; ++i) {
+    if (CPU_ISSET(i, &affinity)) {
+      ++num_cpus;
+    }
+  }
+  return num_cpus;
+#elif defined(BENCHMARK_OS_WINDOWS_WIN32)
+  DWORD_PTR process_mask = 0;
+  DWORD_PTR system_mask = 0;
+  if (GetProcessAffinityMask(GetCurrentProcess(), &process_mask,
+                             &system_mask) == 0) {
+    return 0;
+  }
+  return static_cast<int>(
+      std::bitset<sizeof(DWORD_PTR) * CHAR_BIT>(process_mask).count());
+#else
+  return 0;
+#endif
+}
+
 class ThreadAffinityGuard final {
  public:
   ThreadAffinityGuard() : reset_affinity(SetAffinity()) {
@@ -896,6 +924,7 @@ const CPUInfo& CPUInfo::Get() {
 
 CPUInfo::CPUInfo()
     : num_cpus(GetNumCPUs()),
+      num_cpus_in_affinity_mask(GetNumCPUsInAffinityMask()),
       scaling(CpuScaling(num_cpus)),
       cycles_per_second(GetCPUCyclesPerSecond(scaling)),
       caches(GetCacheSizes()),
diff --git a/test/reporter_output_test.cc b/test/reporter_output_test.cc
index 2ad2e371bd..1f49f5211e 100644
--- a/test/reporter_output_test.cc
+++ b/test/reporter_output_test.cc
@@ -22,7 +22,9 @@ int AddContextCases() {
            {
                {"^%int-%int-%intT%int:%int:%int[-+]%int:%int$", MR_Default},
                {"Running .*(/|\\\\)reporter_output_test(\\.exe)?$", MR_Next},
-               {"Run on \\(%int X %float MHz CPU s?\\)", MR_Next},
+               {"Run on \\(%int X %float MHz CPU s?"
+                "(, %int in affinity mask)?\\)",
+                MR_Next},
            });
   AddCases(TC_JSONOut,
            {{"^\\{", MR_Default},
@@ -31,10 +33,14 @@ int AddContextCases() {
             {"\"host_name\":", MR_Next},
             {"\"executable\": \".*(/|\\\\)reporter_output_test(\\.exe)?\",",
              MR_Next},
-            {"\"num_cpus\": %int,$", MR_Next},
-            {"\"mhz_per_cpu\": %float,$", MR_Next},
-            {"\"caches\": \\[$", MR_Default}});
+            {"\"num_cpus\": %int,$", MR_Next}});
   auto const& Info = benchmark::CPUInfo::Get();
+  // The affinity field is only emitted when it could be determined.
+  if (Info.num_cpus_in_affinity_mask > 0) {
+    AddCases(TC_JSONOut, {{"\"num_cpus_in_affinity_mask\": %int,$", MR_Next}});
+  }
+  AddCases(TC_JSONOut, {{"\"mhz_per_cpu\": %float,$", MR_Next},
+                        {"\"caches\": \\[$", MR_Default}});
   auto const& Caches = Info.caches;
   if (!Caches.empty()) {
     AddCases(TC_ConsoleErr, {{"CPU Caches:$", MR_Next}});