diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5459f1c1cf..6b494ff9da 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -40,6 +40,9 @@ Increment the:
 * Enable WITH_OTLP_RETRY_PREVIEW by default
   [#3953](https://github.com/open-telemetry/opentelemetry-cpp/pull/3953)
 
+* Enable WITH_OTLP_GRPC_SSL_MTLS_PREVIEW by default
+  [#3970](https://github.com/open-telemetry/opentelemetry-cpp/pull/3970)
+
 Important changes:
 
 * Enable WITH_OTLP_RETRY_PREVIEW by default
@@ -48,6 +51,15 @@ Important changes:
   * CMake flag WITH_OTLP_RETRY_PREVIEW is now enabled by default.
   * This flag is deprecated, planned for removal.
 
+* Enable WITH_OTLP_GRPC_SSL_MTLS_PREVIEW by default
+  [#3970](https://github.com/open-telemetry/opentelemetry-cpp/pull/3970)
+
+  * CMake flag WITH_OTLP_GRPC_SSL_MTLS_PREVIEW is now enabled by default.
+  * This flag is deprecated, planned for removal.
+  * Bazel now always builds with ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW.
+  * grpc properties for ssl KEY and CERT are always available,
+    adjust the application code to initialize all members in grpc options.
+
 ## [1.26.0] 2026-03-19
 
 * [RELEASE] Bump main branch to 1.26.0-dev
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 82a2513fb3..614f089501 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -134,7 +134,7 @@ endif()
 option(WITH_OTLP_RETRY_PREVIEW "Whether to enable retry functionality" ON)
 
 option(WITH_OTLP_GRPC_SSL_MTLS_PREVIEW
-       "Whether to enable mTLS support for gRPC" OFF)
+       "Whether to enable mTLS support for gRPC" ON)
 
 option(WITH_OTLP_GRPC_CREDENTIAL_PREVIEW
        "Whether to enable gRPC credentials option in OTLP gRPC Exporter" OFF)
diff --git a/DEPRECATED.md b/DEPRECATED.md
index ad1b759f5c..29971657c7 100644
--- a/DEPRECATED.md
+++ b/DEPRECATED.md
@@ -88,6 +88,58 @@ to enable or disable the retry feature.
 
 This compilation flag will be removed after October 1st, 2026.
 
+### CMake WITH_OTLP_GRPC_SSL_MTLS_PREVIEW
+
+#### Announcement (WITH_OTLP_GRPC_SSL_MTLS_PREVIEW)
+
+The CMake compile flag WITH_OTLP_GRPC_SSL_MTLS_PREVIEW is deprecated by:
+
+* Enable WITH_OTLP_GRPC_SSL_MTLS_PREVIEW by default
+  [#3970](https://github.com/open-telemetry/opentelemetry-cpp/pull/3970)
+
+#### Motivation (WITH_OTLP_GRPC_SSL_MTLS_PREVIEW)
+
+Flags like `WITH_OTLP_GRPC_SSL_MTLS_PREVIEW` are used to conditionally compile
+new features, when they are introduced in the code base.
+
+Now that this feature is stable, conditional compilation is no longer
+necessary.
+
+#### Scope (WITH_OTLP_GRPC_SSL_MTLS_PREVIEW)
+
+Remove compilation flag WITH_OTLP_GRPC_SSL_MTLS_PREVIEW in CMake.
+
+Remove ifdef ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW in C++ .
+
+#### Mitigation (WITH_OTLP_GRPC_SSL_MTLS_PREVIEW)
+
+Applications built with `WITH_OTLP_GRPC_SSL_MTLS_PREVIEW=ON` need to:
+
+* remove the WITH_OTLP_GRPC_SSL_MTLS_PREVIEW flag from CMake scripts
+
+Applications built with `WITH_OTLP_GRPC_SSL_MTLS_PREVIEW=OFF` need to:
+
+* remove the WITH_OTLP_GRPC_SSL_MTLS_PREVIEW flag from CMake scripts
+* initialize ssl client properties in the OTLP GRPC exporter options,
+  either with real KEY and CERT data, or with empty strings.
+
+See:
+
+* struct `OtlpGrpcClientOptions`
+* struct `OtlpGrpcLogRecordExporterOptions`
+* struct `OtlpGrpcMetricExporterOptions`
+* struct `OtlpGrpcExporterOptions`
+
+For Bazel, no compilation flag exists, the code is always built using
+ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW.
+
+Make sure to properly initialize ssl client properties
+to enable or disable the feature.
+
+#### Planned removal (WITH_OTLP_GRPC_SSL_MTLS_PREVIEW)
+
+This compilation flag will be removed after October 1st, 2026.
+
 ## [opentelemetry-cpp API]
 
 ### Deprecation of EventLogger
diff --git a/api/CMakeLists.txt b/api/CMakeLists.txt
index 2e4191a866..069957a251 100644
--- a/api/CMakeLists.txt
+++ b/api/CMakeLists.txt
@@ -91,6 +91,8 @@ endif()
 if(WITH_OTLP_GRPC_SSL_MTLS_PREVIEW)
   target_compile_definitions(opentelemetry_api
                              INTERFACE ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW)
+else()
+  message(WARNING "WITH_OTLP_GRPC_SSL_MTLS_PREVIEW=OFF is deprecated")
 endif()
 
 if(WITH_OTLP_GRPC_CREDENTIAL_PREVIEW)
diff --git a/exporters/otlp/BUILD b/exporters/otlp/BUILD
index 74aa818310..0fb2ee0c19 100644
--- a/exporters/otlp/BUILD
+++ b/exporters/otlp/BUILD
@@ -76,7 +76,7 @@ cc_library(
     defines = select({
         ":enable_otlp_grpc_credential_preview": ["ENABLE_OTLP_GRPC_CREDENTIAL_PREVIEW"],
         "//conditions:default": [],
-    }),
+    }) + ["ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW"],
     strip_include_prefix = "include",
     tags = [
         "otlp",
@@ -122,6 +122,7 @@ cc_library(
         "include/opentelemetry/exporters/otlp/protobuf_include_prefix.h",
         "include/opentelemetry/exporters/otlp/protobuf_include_suffix.h",
     ],
+    defines = ["ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW"],
     strip_include_prefix = "include",
     tags = [
         "otlp",
@@ -146,6 +147,7 @@ cc_library(
     hdrs = [
         "include/opentelemetry/exporters/otlp/otlp_grpc_span_builder.h",
     ],
+    defines = ["ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW"],
     strip_include_prefix = "include",
     tags = [
         "otlp",
@@ -336,6 +338,7 @@ cc_library(
         "include/opentelemetry/exporters/otlp/protobuf_include_prefix.h",
         "include/opentelemetry/exporters/otlp/protobuf_include_suffix.h",
     ],
+    defines = ["ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW"],
     strip_include_prefix = "include",
     tags = [
         "otlp",
@@ -360,6 +363,7 @@ cc_library(
     hdrs = [
         "include/opentelemetry/exporters/otlp/otlp_grpc_push_metric_builder.h",
     ],
+    defines = ["ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW"],
     strip_include_prefix = "include",
     tags = [
         "otlp",
@@ -602,6 +606,7 @@ cc_library(
     hdrs = [
         "include/opentelemetry/exporters/otlp/otlp_grpc_log_record_builder.h",
     ],
+    defines = ["ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW"],
     strip_include_prefix = "include",
     tags = [
         "otlp",
diff --git a/exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_client_options.h b/exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_client_options.h
index f377ef7d09..baec1350e7 100644
--- a/exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_client_options.h
+++ b/exporters/otlp/include/opentelemetry/exporters/otlp/otlp_grpc_client_options.h
@@ -42,7 +42,6 @@ struct OtlpGrpcClientOptions
   /** CA CERT, as a string. */
   std::string ssl_credentials_cacert_as_string;
 
-#ifdef ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW
   /** CLIENT KEY, path to a file. */
   std::string ssl_client_key_path;
 
@@ -54,7 +53,6 @@ struct OtlpGrpcClientOptions
 
   /** CLIENT CERT, as a string. */
   std::string ssl_client_cert_string;
-#endif
 
 #ifdef ENABLE_OTLP_GRPC_CREDENTIAL_PREVIEW
   /** Use custom ChannelCredentials, instead of the SSL options above. */