diff --git a/crypto/internal.h b/crypto/internal.h
index 0e8ae3a6..6944d56a 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -132,13 +132,13 @@
 #endif
 #endif
 
-#if !defined(OPENSSL_NO_THREADS) && \
+#if defined(OPENSSL_THREADS) && \
     (!defined(OPENSSL_WINDOWS) || defined(__MINGW32__))
 #include <pthread.h>
 #define OPENSSL_PTHREADS
 #endif
 
-#if !defined(OPENSSL_NO_THREADS) && !defined(OPENSSL_PTHREADS) && \
+#if defined(OPENSSL_THREADS) && !defined(OPENSSL_PTHREADS) && \
     defined(OPENSSL_WINDOWS)
 #define OPENSSL_WINDOWS_THREADS
 OPENSSL_MSVC_PRAGMA(warning(push, 3))
@@ -367,7 +367,7 @@ static inline int constant_time_select_int(crypto_word_t mask, int a, int b) {
 
 // Thread-safe initialisation.
 
-#if defined(OPENSSL_NO_THREADS)
+#if !defined(OPENSSL_THREADS)
 typedef uint32_t CRYPTO_once_t;
 #define CRYPTO_ONCE_INIT 0
 #elif defined(OPENSSL_WINDOWS_THREADS)
@@ -423,7 +423,7 @@ OPENSSL_EXPORT int CRYPTO_refcount_dec_and_test_zero(CRYPTO_refcount_t *count);
 // thread.h as a structure large enough to fit the real type. The global lock is
 // a different type so it may be initialized with platform initializer macros.
 
-#if defined(OPENSSL_NO_THREADS)
+#if !defined(OPENSSL_THREADS)
 struct CRYPTO_STATIC_MUTEX {
   char padding;  // Empty structs have different sizes in C and C++.
 };
diff --git a/crypto/pool/pool_test.cc b/crypto/pool/pool_test.cc
index 62fb17dd..8f32fb6f 100644
--- a/crypto/pool/pool_test.cc
+++ b/crypto/pool/pool_test.cc
@@ -18,7 +18,7 @@
 
 #include "../test/test_util.h"
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 #include <chrono>
 #include <thread>
 #endif
@@ -61,7 +61,7 @@ TEST(PoolTest, Pooled) {
   EXPECT_EQ(buf.get(), buf2.get()) << "CRYPTO_BUFFER_POOL did not dedup data.";
 }
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 TEST(PoolTest, Threads) {
   bssl::UniquePtr<CRYPTO_BUFFER_POOL> pool(CRYPTO_BUFFER_POOL_new());
   ASSERT_TRUE(pool);
diff --git a/crypto/rand_extra/rand_test.cc b/crypto/rand_extra/rand_test.cc
index bd2eb188..183c4f77 100644
--- a/crypto/rand_extra/rand_test.cc
+++ b/crypto/rand_extra/rand_test.cc
@@ -20,7 +20,7 @@
 
 #include "../test/test_util.h"
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 #include <array>
 #include <thread>
 #include <vector>
@@ -146,7 +146,7 @@ TEST(RandTest, Fork) {
 }
 #endif  // !OPENSSL_WINDOWS && !BORINGSSL_UNSAFE_DETERMINISTIC_MODE
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 static void RunConcurrentRands(size_t num_threads) {
   static const uint8_t kZeros[256] = {0};
 
diff --git a/crypto/refcount_test.cc b/crypto/refcount_test.cc
index 6ce0746d..700863f6 100644
--- a/crypto/refcount_test.cc
+++ b/crypto/refcount_test.cc
@@ -16,7 +16,7 @@
 
 #include <gtest/gtest.h>
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 #include <thread>
 #endif
 
@@ -43,7 +43,7 @@ TEST(RefCountTest, Basic) {
   EXPECT_EQ(1u, count);
 }
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 // This test is primarily intended to run under ThreadSanitizer.
 TEST(RefCountTest, Threads) {
   CRYPTO_refcount_t count = 0;
diff --git a/crypto/rsa_extra/rsa_test.cc b/crypto/rsa_extra/rsa_test.cc
index b0a0b7e4..9bd47b1b 100644
--- a/crypto/rsa_extra/rsa_test.cc
+++ b/crypto/rsa_extra/rsa_test.cc
@@ -72,7 +72,7 @@
 #include "../internal.h"
 #include "../test/test_util.h"
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 #include <thread>
 #include <vector>
 #endif
@@ -1048,7 +1048,7 @@ TEST(RSATest, SqrtTwo) {
 }
 #endif  // !BORINGSSL_SHARED_LIBRARY
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 TEST(RSATest, Threads) {
   bssl::UniquePtr<RSA> rsa_template(
       RSA_private_key_from_bytes(kKey1, sizeof(kKey1) - 1));
diff --git a/crypto/thread_none.c b/crypto/thread_none.c
index 718d9601..4f07b9d9 100644
--- a/crypto/thread_none.c
+++ b/crypto/thread_none.c
@@ -14,7 +14,7 @@
 
 #include "internal.h"
 
-#if defined(OPENSSL_NO_THREADS)
+#if !defined(OPENSSL_THREADS)
 
 void CRYPTO_MUTEX_init(CRYPTO_MUTEX *lock) {}
 
@@ -56,4 +56,4 @@ int CRYPTO_set_thread_local(thread_local_data_t index, void *value,
   return 1;
 }
 
-#endif  // OPENSSL_NO_THREADS
+#endif  // !OPENSSL_THREADS
diff --git a/crypto/thread_test.cc b/crypto/thread_test.cc
index f89b22ec..f9fad9be 100644
--- a/crypto/thread_test.cc
+++ b/crypto/thread_test.cc
@@ -25,7 +25,7 @@
 #include "test/test_util.h"
 
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 
 static unsigned g_once_init_called = 0;
 
@@ -130,4 +130,4 @@ TEST(ThreadTest, RandState) {
   thread.join();
 }
 
-#endif  // !OPENSSL_NO_THREADS
+#endif  // OPENSSL_THREADS
diff --git a/include/openssl/base.h b/include/openssl/base.h
index d1349934..e287de73 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -136,14 +136,36 @@ extern "C" {
 
 #if defined(TRUSTY)
 #define OPENSSL_TRUSTY
-#define OPENSSL_NO_THREADS
+#define OPENSSL_NO_THREADS_CORRUPT_MEMORY_AND_LEAK_SECRETS_IF_THREADED
 #endif
 
 #if defined(__ANDROID_API__)
 #define OPENSSL_ANDROID
 #endif
 
-#if !defined(OPENSSL_NO_THREADS)
+// OPENSSL_NO_THREADS has been deprecated in favor of this much longer and
+// louder name, to better reflect exactly what that option did.
+//
+// TODO(davidben): Remove this block when callers have migrated.
+#if defined(OPENSSL_NO_THREADS) && \
+    !defined(OPENSSL_NO_THREADS_CORRUPT_MEMORY_AND_LEAK_SECRETS_IF_THREADED)
+#define OPENSSL_NO_THREADS_CORRUPT_MEMORY_AND_LEAK_SECRETS_IF_THREADED
+#endif
+
+// BoringSSL requires platform's locking APIs to make internal global state
+// thread-safe, including the PRNG. On some single-threaded embedded platforms,
+// locking APIs may not exist, so this dependency may be disabled with the
+// following build flag.
+//
+// IMPORTANT: Doing so means the consumer promises the library will never be
+// used in any multi-threaded context. It causes BoringSSL to be globally
+// thread-unsafe. Setting it inappropriately will subtly and unpredictably
+// corrupt memory and leak secret keys.
+//
+// Do not set this flag on any platform where threads are possible. BoringSSL
+// maintainers will not provide support for any consumers that do so. Changes
+// which break such unsupported configurations will not be reverted.
+#if !defined(OPENSSL_NO_THREADS_CORRUPT_MEMORY_AND_LEAK_SECRETS_IF_THREADED)
 #define OPENSSL_THREADS
 #endif
 
diff --git a/include/openssl/thread.h b/include/openssl/thread.h
index 98073b07..91706fec 100644
--- a/include/openssl/thread.h
+++ b/include/openssl/thread.h
@@ -66,7 +66,7 @@ extern "C" {
 #endif
 
 
-#if defined(OPENSSL_NO_THREADS)
+#if !defined(OPENSSL_THREADS)
 typedef struct crypto_mutex_st {
   char padding;  // Empty structs have different sizes in C and C++.
 } CRYPTO_MUTEX;
diff --git a/ssl/ssl_test.cc b/ssl/ssl_test.cc
index 894bb14e..61a47d3d 100644
--- a/ssl/ssl_test.cc
+++ b/ssl/ssl_test.cc
@@ -48,7 +48,7 @@ OPENSSL_MSVC_PRAGMA(warning(pop))
 #include <sys/time.h>
 #endif
 
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 #include <thread>
 #endif
 
@@ -4298,7 +4298,7 @@ TEST_P(SSLVersionTest, FakeIDsForTickets) {
 
 // These tests test multi-threaded behavior. They are intended to run with
 // ThreadSanitizer.
-#if !defined(OPENSSL_NO_THREADS)
+#if defined(OPENSSL_THREADS)
 TEST_P(SSLVersionTest, SessionCacheThreads) {
   SSL_CTX_set_options(server_ctx_.get(), SSL_OP_NO_TICKET);
   SSL_CTX_set_session_cache_mode(client_ctx_.get(), SSL_SESS_CACHE_BOTH);