diff --git a/src/compiler/propagation/type_primitive_core.cc b/src/compiler/propagation/type_primitive_core.cc
index 41935090c..95c01db5b 100644
--- a/src/compiler/propagation/type_primitive_core.cc
+++ b/src/compiler/propagation/type_primitive_core.cc
@@ -270,6 +270,7 @@ TYPE_PRIMITIVE_ANY(debug_set_memory_limit)
 TYPE_PRIMITIVE_ANY(dump_heap)
 TYPE_PRIMITIVE_ANY(serial_print_heap_report)
 TYPE_PRIMITIVE_ANY(get_env)
+TYPE_PRIMITIVE_NULL(set_env)
 TYPE_PRIMITIVE_ANY(literal_index)
 TYPE_PRIMITIVE_ANY(firmware_map)
 TYPE_PRIMITIVE_ANY(firmware_unmap)
diff --git a/src/os.h b/src/os.h
index f0066ff7b..cc2b4fc87 100644
--- a/src/os.h
+++ b/src/os.h
@@ -241,7 +241,11 @@ class OS {
   static word get_heap_tag();
   static void heap_summary_report(int max_pages, const char* marker);
 
-  static const char* getenv(const char* variable);
+  // Returns a malloced string.
+  static char* getenv(const char* variable);
+  // Returns true for OK.
+  static bool setenv(const char* variable, const char* value);
+  static bool unsetenv(const char* variable);
 
 #ifdef TOIT_FREERTOS
   static bool use_spiram_for_heap() { return use_spiram_for_heap_; }
diff --git a/src/os_esp32.cc b/src/os_esp32.cc
index 726d61a9a..0f67142af 100644
--- a/src/os_esp32.cc
+++ b/src/os_esp32.cc
@@ -800,7 +800,7 @@ void OS::heap_summary_report(int max_pages, const char* marker) {}
 
 #endif // def TOIT_CMPCTMALLOC
 
-const char* OS::getenv(const char* variable) {
+char* OS::getenv(const char* variable) {
   // Unimplemented on purpose.
   // We currently prefer not to expose environment variables on embedded devices.
   // There is no technical reason for it, so if circumstances change, one can
@@ -808,6 +808,16 @@ const char* OS::getenv(const char* variable) {
   UNIMPLEMENTED();
 }
 
+bool OS::setenv(const char* variable, const char* value) {
+  // Unimplemented on purpose.
+  UNIMPLEMENTED();
+}
+
+bool OS::unsetenv(const char* variable) {
+  // Unimplemented on purpose.
+  UNIMPLEMENTED();
+}
+
 bool OS::set_real_time(struct timespec* time) {
   if (clock_settime(CLOCK_REALTIME, time) == 0) return true;
   struct timeval timeofday{};
diff --git a/src/os_posix.cc b/src/os_posix.cc
index 579c8d31e..96dcec4bd 100644
--- a/src/os_posix.cc
+++ b/src/os_posix.cc
@@ -255,8 +255,22 @@ void OS::out_of_memory(const char* reason) {
   abort();
 }
 
-const char* OS::getenv(const char* variable) {
-  return ::getenv(variable);
+char* OS::getenv(const char* variable) {
+  // Getenv/setenv are not guaranteed to be reentrant.
+  Locker scope(global_mutex_);
+  char* result = ::getenv(variable);
+  if (result == null) return null;
+  return strdup(result);
+}
+
+bool OS::setenv(const char* variable, const char* value) {
+  Locker scope(global_mutex_);
+  return ::setenv(variable, value, 1) == 0;
+}
+
+bool OS::unsetenv(const char* variable) {
+  Locker scope(global_mutex_);
+  return ::unsetenv(variable) == 0;
 }
 
 bool OS::set_real_time(struct timespec* time) {
diff --git a/src/os_win.cc b/src/os_win.cc
index 2515e63e5..08281af58 100644
--- a/src/os_win.cc
+++ b/src/os_win.cc
@@ -256,8 +256,43 @@ void OS::out_of_memory(const char* reason) {
   abort();
 }
 
-const char* OS::getenv(const char* variable) {
-  return ::getenv(variable);
+static wchar_t* malloced_wide_string(const char* string) {
+  word length = Utils::utf_8_to_16(unsigned_cast(string), strlen(string));
+  wchar_t* result = reinterpret_cast<wchar_t*>(malloc((length + 1) * sizeof(wchar_t)));
+  Utils::utf_8_to_16(unsigned_cast(string), strlen(string), result, length);
+  result[length] = '\0';
+  return result;
+}
+
+char* OS::getenv(const char* variable) {
+  wchar_t* wide_variable = malloced_wide_string(variable);
+
+  const int BUFFER_SIZE = 32767;
+  wchar_t buffer[BUFFER_SIZE];
+  int wide_length = GetEnvironmentVariableW(wide_variable, buffer, BUFFER_SIZE);
+  free(wide_variable);
+  if (wide_length == 0 || wide_length > BUFFER_SIZE) return null;
+  word size = Utils::utf_16_to_8(buffer, wide_length, null, 0);
+  char* result = unvoid_cast<char*>(malloc(size + 1));
+  Utils::utf_16_to_8(buffer, wide_length, unsigned_cast(result), size);
+  result[size] = '\0';
+  return result;
+}
+
+bool OS::setenv(const char* variable, const char* value) {
+  wchar_t* wide_variable = malloced_wide_string(variable);
+  wchar_t* wide_value = malloced_wide_string(value);
+  bool ok = SetEnvironmentVariableW(wide_variable, wide_value);
+  free(wide_variable);
+  free(wide_value);
+  return ok;
+}
+
+bool OS::unsetenv(const char* variable) {
+  wchar_t* wide_variable = malloced_wide_string(variable);
+  bool ok = SetEnvironmentVariableW(wide_variable, null);
+  free(wide_variable);
+  return ok;
 }
 
 bool OS::set_real_time(struct timespec* time) {
diff --git a/src/primitive.h b/src/primitive.h
index 6419a7830..3db07a2b3 100644
--- a/src/primitive.h
+++ b/src/primitive.h
@@ -237,6 +237,7 @@ namespace toit {
   PRIMITIVE(dump_heap, 1)                    \
   PRIMITIVE(serial_print_heap_report, 2)     \
   PRIMITIVE(get_env, 1)                      \
+  PRIMITIVE(set_env, 2)                      \
   PRIMITIVE(literal_index, 1)                \
   PRIMITIVE(word_size, 0)                    \
   PRIMITIVE(firmware_map, 1)                 \
diff --git a/src/primitive_core.cc b/src/primitive_core.cc
index 3f10815ff..9ee95bdfa 100644
--- a/src/primitive_core.cc
+++ b/src/primitive_core.cc
@@ -2244,7 +2244,7 @@ PRIMITIVE(dump_heap) {
 #else
   ARGS(int, padding);
   if (padding < 0 || padding > 0x10000) OUT_OF_RANGE;
-#if defined(TOIT_LINUX)
+#ifdef TOIT_LINUX
   if (heap_caps_iterate_tagged_memory_areas == null) {
     // This always happens on the server unless we are running with
     // cmpctmalloc (using LD_PRELOAD), which supports iterating the heap in
@@ -2290,16 +2290,31 @@ PRIMITIVE(serial_print_heap_report) {
 }
 
 PRIMITIVE(get_env) {
-#if defined (TOIT_FREERTOS)
+#ifdef TOIT_FREERTOS
   // FreeRTOS supports environment variables, but we prefer not to expose them.
   UNIMPLEMENTED_PRIMITIVE;
 #else
   ARGS(cstring, key);
-  // TODO(florian): getenv is not reentrant.
-  //   We should have a lock around `getenv` and `setenv`.
-  const char* result = OS::getenv(key);
+  char* result = OS::getenv(key);
   if (result == null) return process->program()->null_object();
-  return process->allocate_string_or_error(result, strlen(result));
+  Object* string_or_error = process->allocate_string_or_error(result, strlen(result));
+  free(result);
+  return string_or_error;
+#endif
+}
+
+PRIMITIVE(set_env) {
+#ifdef TOIT_FREERTOS
+  // FreeRTOS supports environment variables, but we prefer not to expose them.
+  UNIMPLEMENTED_PRIMITIVE;
+#else
+  ARGS(cstring, key, cstring, value);
+  if (value) {
+    OS::setenv(key, value);
+  } else {
+    OS::unsetenv(key);
+  }
+  return process->program()->null_object();
 #endif
 }