diff --git a/ChangeLog.md b/ChangeLog.md
index 817629e1f8f9e..884dacf21ab05 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -39,6 +39,10 @@ See docs/process.md for more on how version tagging works.
   functions. (#26297)
 - libpng port updated from 1.6.39 to 1.6.55. (#26388)
 - Added sdl3_ttf port. (#24601)
+- The `STRICT_JS` setting is now on by default.  Previously it was enabled by
+  default in `STRICT` mode.  If you have EM_ASM or EM_JS or `--pre-js` code
+  that does not conform to JS strict mode then you may need to disable this
+  with `-sSTRICT_JS=0`. (#26421)
 
 5.0.2 - 02/25/26
 ----------------
diff --git a/site/source/docs/tools_reference/settings_reference.rst b/site/source/docs/tools_reference/settings_reference.rst
index 89f79600b845c..26648970ac345 100644
--- a/site/source/docs/tools_reference/settings_reference.rst
+++ b/site/source/docs/tools_reference/settings_reference.rst
@@ -1703,7 +1703,6 @@ Changes enabled by this:
 
   - The C define EMSCRIPTEN is not defined (__EMSCRIPTEN__ always is, and
     is the correct thing to use).
-  - STRICT_JS is enabled.
   - IGNORE_MISSING_MAIN is disabled.
   - AUTO_JS_LIBRARIES is disabled.
   - AUTO_NATIVE_LIBRARIES is disabled.
@@ -1734,7 +1733,7 @@ STRICT_JS
 
 Add ``"use strict;"`` to generated JS
 
-Default value: false
+Default value: true
 
 .. _warn_on_undefined_symbols:
 
diff --git a/src/settings.js b/src/settings.js
index e7148433a5f94..57466bbd4f34e 100644
--- a/src/settings.js
+++ b/src/settings.js
@@ -1162,7 +1162,6 @@ var LINKABLE = false;
 //
 //   - The C define EMSCRIPTEN is not defined (__EMSCRIPTEN__ always is, and
 //     is the correct thing to use).
-//   - STRICT_JS is enabled.
 //   - IGNORE_MISSING_MAIN is disabled.
 //   - AUTO_JS_LIBRARIES is disabled.
 //   - AUTO_NATIVE_LIBRARIES is disabled.
@@ -1181,7 +1180,7 @@ var IGNORE_MISSING_MAIN = true;
 
 // Add ``"use strict;"`` to generated JS
 // [link]
-var STRICT_JS = false;
+var STRICT_JS = true;
 
 // If set to 1, we will warn on any undefined symbols that are not resolved by
 // the ``library_*.js`` files. Note that it is common in large projects to not
diff --git a/test/codesize/test_unoptimized_code_size.json b/test/codesize/test_unoptimized_code_size.json
index 635a347810bec..e03c36608f744 100644
--- a/test/codesize/test_unoptimized_code_size.json
+++ b/test/codesize/test_unoptimized_code_size.json
@@ -1,16 +1,16 @@
 {
-  "hello_world.js": 57007,
-  "hello_world.js.gz": 17736,
+  "hello_world.js": 57022,
+  "hello_world.js.gz": 17744,
   "hello_world.wasm": 15138,
   "hello_world.wasm.gz": 7455,
-  "no_asserts.js": 26576,
-  "no_asserts.js.gz": 8881,
+  "no_asserts.js": 26591,
+  "no_asserts.js.gz": 8894,
   "no_asserts.wasm": 12187,
   "no_asserts.wasm.gz": 5984,
   "strict.js": 54825,
   "strict.js.gz": 17039,
   "strict.wasm": 15138,
   "strict.wasm.gz": 7450,
-  "total": 180871,
-  "total_gz": 64545
+  "total": 180901,
+  "total_gz": 64566
 }
diff --git a/test/test_browser.py b/test/test_browser.py
index 234d24b305287..d0f63f7e803d0 100644
--- a/test/test_browser.py
+++ b/test/test_browser.py
@@ -945,7 +945,7 @@ def test_sdl_stb_image_cleanup(self):
     'safe_heap_O2': (['-sSAFE_HEAP', '-O2'],),
   })
   def test_sdl_canvas(self, args):
-    self.btest_exit('test_sdl_canvas.c', cflags=['-sSTRICT_JS', '-sLEGACY_GL_EMULATION', '-lSDL', '-lGL'] + args)
+    self.btest_exit('test_sdl_canvas.c', cflags=['-sLEGACY_GL_EMULATION', '-lSDL', '-lGL'] + args)
 
   def test_sdl_canvas_alpha(self):
     # N.B. On Linux with Intel integrated graphics cards, this test needs Firefox 49 or newer.
@@ -1059,7 +1059,7 @@ def test_sdl_mouse_offsets(self):
     self.run_browser('page.html', '', '/report_result?exit:0')
 
   def test_glut_touchevents(self):
-    self.btest_exit('glut_touchevents.c', cflags=['-lglut', '-sSTRICT_JS'])
+    self.btest_exit('glut_touchevents.c', cflags=['-lglut'])
 
   def test_glut_wheelevents(self):
     self.btest_exit('glut_wheelevents.c', cflags=['-lglut'])
@@ -4953,7 +4953,6 @@ def test_no_declare_asm_module_exports_wasm2js(self, args):
 
   @parameterized({
     '': ([],),
-    'strict_js': (['-sSTRICT_JS'],),
     'minimal_runtime': (['-sMINIMAL_RUNTIME=1'],),
     'minimal_runtime_2': (['-sMINIMAL_RUNTIME=2'],),
   })
@@ -5416,8 +5415,8 @@ def test_pthread_unhandledrejection(self):
   def test_pthread_key_recreation(self):
     self.btest_exit('pthread/test_pthread_key_recreation.c', cflags=['-pthread', '-sPTHREAD_POOL_SIZE=1'])
 
-  def test_full_js_library_strict(self):
-    self.btest_exit('hello_world.c', cflags=['-sINCLUDE_FULL_LIBRARY', '-sSTRICT_JS'])
+  def test_full_js_library(self):
+    self.btest_exit('hello_world.c', cflags=['-sINCLUDE_FULL_LIBRARY'])
 
   # Tests the AudioWorklet demo
   @parameterized({
diff --git a/test/test_core.py b/test/test_core.py
index c517d88fb4552..2b9d7593e1e44 100644
--- a/test/test_core.py
+++ b/test/test_core.py
@@ -429,7 +429,6 @@ def decorated(self, textdecoder, *args, **kwargs):
 
 no_safe_heap = make_no_decorator_for_setting('SAFE_HEAP')
 no_strict = make_no_decorator_for_setting('STRICT')
-no_strict_js = make_no_decorator_for_setting('STRICT_JS')
 no_big_endian = make_no_decorator_for_setting('SUPPORT_BIG_ENDIAN')
 no_omit_asm_module_exports = make_no_decorator_for_setting('DECLARE_ASM_MODULE_EXPORTS=0')
 no_js_math = make_no_decorator_for_setting('JS_MATH')
@@ -9714,7 +9713,6 @@ def test_esm_integration(self):
     self.assertFileContents(test_file('core/test_esm_integration.expected.mjs'), read_file('hello_world.mjs'))
 
   @no_omit_asm_module_exports('MODULARIZE is not compatible with DECLARE_ASM_MODULE_EXPORTS=0')
-  @no_strict_js('EXPORT_ES6 is not compatible with STRICT_JS')
   def test_modularize_instance_hello(self):
     self.do_core_test('test_hello_world.c', cflags=['-sMODULARIZE=instance', '-Wno-experimental'])
 
@@ -9723,7 +9721,6 @@ def test_modularize_instance_hello(self):
     'pthreads': (['-pthread'],),
   })
   @no_omit_asm_module_exports('MODULARIZE is not compatible with DECLARE_ASM_MODULE_EXPORTS=0')
-  @no_strict_js('EXPORT_ES6 is not compatible with STRICT_JS')
   def test_modularize_instance(self, args):
     if args:
       self.require_pthreads()
@@ -9759,7 +9756,6 @@ def test_modularize_instance(self, args):
 
   @no_omit_asm_module_exports('MODULARIZE is not compatible with DECLARE_ASM_MODULE_EXPORTS=0')
   @no_4gb('EMBIND_AOT can\'t lower 4gb')
-  @no_strict_js('EXPORT_ES6 is not compatible with STRICT_JS')
   def test_modularize_instance_embind(self):
     self.run_process([EMXX, test_file('modularize_instance_embind.cpp'),
                       '-sMODULARIZE=instance',
@@ -9940,7 +9936,6 @@ def setUp(self):
 
 # Add DEFAULT_TO_CXX=0
 strict = make_run('strict', cflags=[], settings={'STRICT': 1})
-strict_js = make_run('strict_js', cflags=[], settings={'STRICT_JS': 1})
 
 ubsan = make_run('ubsan', cflags=['-fsanitize=undefined', '--profiling'])
 lsan = make_run('lsan', cflags=['-fsanitize=leak', '--profiling'], settings={'ALLOW_MEMORY_GROWTH': 1})
diff --git a/test/test_other.py b/test/test_other.py
index 971eea354e4bd..3fa31ceb5d9f1 100644
--- a/test/test_other.py
+++ b/test/test_other.py
@@ -3374,7 +3374,6 @@ def test_embind_subclass_pointer(self):
     'o2': ['-O2'],
     'o2_mem_growth': ['-O2', '-sALLOW_MEMORY_GROWTH', test_file('embind/isMemoryGrowthEnabled=true.cpp')],
     'o2_closure': ['-O2', '--closure=1', '--closure-args', '--externs ' + shlex.quote(test_file('embind/underscore-externs.js')), '-sASSERTIONS=1'],
-    'strict_js': ['-sSTRICT_JS'],
     # DYNCALLS tests the legacy native function API (ASYNCIFY implicitly enables DYNCALLS)
     'dyncalls': ['-sDYNCALLS=1'],
   })
@@ -9005,11 +9004,11 @@ def test(check, extra):
     'embind': (['-lembind'],),
   })
   def test_full_js_library(self, args):
-    self.run_process([EMCC, test_file('hello_world.c'), '-sSTRICT_JS', '-sINCLUDE_FULL_LIBRARY'] + args)
+    self.run_process([EMCC, test_file('hello_world.c'), '-sINCLUDE_FULL_LIBRARY'] + args)
 
   def test_full_js_library_undefined(self):
     create_file('main.c', 'void foo(); int main() { foo(); return 0; }')
-    self.assert_fail([EMCC, 'main.c', '-sSTRICT_JS', '-sINCLUDE_FULL_LIBRARY'], 'undefined symbol: foo')
+    self.assert_fail([EMCC, 'main.c', '-sINCLUDE_FULL_LIBRARY'], 'undefined symbol: foo')
 
   def test_full_js_library_except(self):
     self.set_setting('INCLUDE_FULL_LIBRARY', 1)
@@ -13753,9 +13752,6 @@ def test_std_filesystem_tempdir(self):
       self.skipTest('https://github.com/emscripten-core/emscripten/issues/24830')
     self.do_other_test('test_std_filesystem_tempdir.cpp', cflags=['-g'])
 
-  def test_strict_js_closure(self):
-    self.do_runf('hello_world.c', cflags=['-sSTRICT_JS', '-Werror=closure', '--closure=1', '-O3'])
-
   def test_em_js_deps(self):
     # Check that EM_JS_DEPS works. Specifically, multiple different instances in different
     # object files.
diff --git a/tools/link.py b/tools/link.py
index bf8557e3ea27b..376cc8c0c4bf2 100644
--- a/tools/link.py
+++ b/tools/link.py
@@ -1147,9 +1147,10 @@ def limit_incoming_module_api():
   elif options.shell_html:
     diagnostics.warning('unused-command-line-argument', '--shell-file ignored when not generating html output')
 
+  if settings.EXPORT_ES6:
+    default_setting('STRICT_JS', 0)
+
   if settings.STRICT:
-    if not settings.EXPORT_ES6:
-      default_setting('STRICT_JS', 1)
     default_setting('DEFAULT_TO_CXX', 0)
     default_setting('IGNORE_MISSING_MAIN', 0)
     default_setting('AUTO_NATIVE_LIBRARIES', 0)