diff --git a/esphome/components/web_server/__init__.py b/esphome/components/web_server/__init__.py
index 2587d13b9e0..c1887cc3fc2 100644
--- a/esphome/components/web_server/__init__.py
+++ b/esphome/components/web_server/__init__.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import base64
 import gzip
 import logging
 import re
@@ -406,10 +407,21 @@ async def to_code(config):
         # The scheme is fixed at build time so the unused Basic/Digest code path is compiled
         # out. Basic is the current default (the absence of this define); an explicit
         # 'type: digest' opts in early. Default changes to digest in 2027.1.0.
-        if auth.get(CONF_TYPE) == AUTH_TYPE_DIGEST:
+        is_digest = auth.get(CONF_TYPE) == AUTH_TYPE_DIGEST
+        if is_digest:
             cg.add_define("USE_WEBSERVER_AUTH_DIGEST")
-        cg.add(paren.set_auth_username(auth[CONF_USERNAME]))
-        cg.add(paren.set_auth_password(auth[CONF_PASSWORD]))
+        if is_digest or CORE.is_esp32:
+            cg.add(paren.set_auth_username(auth[CONF_USERNAME]))
+            cg.add(paren.set_auth_password(auth[CONF_PASSWORD]))
+        else:
+            # Every non-ESP32 basic auth build takes this path. The ESP8266 and RP2040
+            # core base64 encoders wrap output every 72 chars, which breaks
+            # ESPAsyncWebServer's basic auth compare for long credentials.
+            # Precompute the hash here and let C++ compare the raw header payload.
+            basic_hash = base64.b64encode(
+                f"{auth[CONF_USERNAME]}:{auth[CONF_PASSWORD]}".encode()
+            ).decode()
+            cg.add(paren.set_auth_basic_hash(basic_hash))
     if CONF_CSS_INCLUDE in config:
         cg.add_define("USE_WEBSERVER_CSS_INCLUDE")
         path = CORE.relative_config_path(config[CONF_CSS_INCLUDE])
diff --git a/esphome/components/web_server_base/web_server_base.cpp b/esphome/components/web_server_base/web_server_base.cpp
index ccfc04f674b..873c5b5a497 100644
--- a/esphome/components/web_server_base/web_server_base.cpp
+++ b/esphome/components/web_server_base/web_server_base.cpp
@@ -7,7 +7,7 @@ WebServerBase *global_web_server_base = nullptr;  // NOLINT(cppcoreguidelines-av
 
 void WebServerBase::add_handler(AsyncWebHandler *handler) {
 #ifdef USE_WEBSERVER_AUTH
-  if (!credentials_.username.empty()) {
+  if (credentials_.is_set()) {
     handler = new internal::AuthMiddlewareHandler(handler, &credentials_);
   }
 #endif
diff --git a/esphome/components/web_server_base/web_server_base.h b/esphome/components/web_server_base/web_server_base.h
index 9657853a73d..c647a13b50b 100644
--- a/esphome/components/web_server_base/web_server_base.h
+++ b/esphome/components/web_server_base/web_server_base.h
@@ -1,7 +1,6 @@
 #pragma once
 #include "esphome/core/defines.h"
 #if defined(USE_NETWORK) && !defined(USE_ZEPHYR)
-#include <utility>
 #include <vector>
 
 #include "esphome/core/progmem.h"
@@ -46,9 +45,20 @@ class MiddlewareHandler : public AsyncWebHandler {
 };
 
 #ifdef USE_WEBSERVER_AUTH
+// All fields point to string literals in generated code; nothing is copied.
 struct Credentials {
-  std::string username;
-  std::string password;
+#if USE_ESP32 || defined(USE_WEBSERVER_AUTH_DIGEST)
+  const char *username{nullptr};
+  const char *password{nullptr};
+  bool is_set() const { return username != nullptr; }
+#else
+  // base64("username:password"), precomputed at codegen time. Used by every non-ESP32 basic
+  // auth build. The ESP8266 and RP2040 core libb64 wraps base64 output every 72 chars, so
+  // letting the library encode and compare fails for long credentials; instead the header
+  // payload is compared against this hash.
+  const char *basic_auth_hash{nullptr};
+  bool is_set() const { return basic_auth_hash != nullptr; }
+#endif
 };
 
 class AuthMiddlewareHandler : public MiddlewareHandler {
@@ -57,10 +67,14 @@ class AuthMiddlewareHandler : public MiddlewareHandler {
       : MiddlewareHandler(next), credentials_(credentials) {}
 
   bool check_auth(AsyncWebServerRequest *request) {
-    bool success = request->authenticate(credentials_->username.c_str(), credentials_->password.c_str());
+    // The scheme is chosen at build time (USE_WEBSERVER_AUTH_DIGEST); the unused path is
+    // compiled out. On ESP32 our own server picks the scheme internally.
+#if USE_ESP32 || defined(USE_WEBSERVER_AUTH_DIGEST)
+    bool success = request->authenticate(credentials_->username, credentials_->password);
+#else
+    bool success = request->authenticate(credentials_->basic_auth_hash);
+#endif
     if (!success) {
-      // The scheme is chosen at build time (USE_WEBSERVER_AUTH_DIGEST); the unused path is
-      // compiled out. On ESP32 our own server picks the scheme internally.
 #if USE_ESP32
       request->requestAuthentication();
 #elif defined(USE_WEBSERVER_AUTH_DIGEST)
@@ -125,8 +139,12 @@ class WebServerBase final {
   AsyncWebServer *get_server() const { return this->server_; }
 
 #ifdef USE_WEBSERVER_AUTH
-  void set_auth_username(std::string auth_username) { credentials_.username = std::move(auth_username); }
-  void set_auth_password(std::string auth_password) { credentials_.password = std::move(auth_password); }
+#if USE_ESP32 || defined(USE_WEBSERVER_AUTH_DIGEST)
+  void set_auth_username(const char *auth_username) { credentials_.username = auth_username; }
+  void set_auth_password(const char *auth_password) { credentials_.password = auth_password; }
+#else
+  void set_auth_basic_hash(const char *hash) { credentials_.basic_auth_hash = hash; }
+#endif
 #endif
 
   void add_handler(AsyncWebHandler *handler);
diff --git a/tests/component_tests/web_server/test_web_server_auth.py b/tests/component_tests/web_server/test_web_server_auth.py
index 82635b26da8..183c586a36e 100644
--- a/tests/component_tests/web_server/test_web_server_auth.py
+++ b/tests/component_tests/web_server/test_web_server_auth.py
@@ -33,14 +33,49 @@ def test_web_server_auth_explicit_basic_no_warning(
     generate_main: Callable[[str], str],
     caplog: pytest.LogCaptureFixture,
 ) -> None:
-    """Auth type basic builds Basic and does not warn."""
-    generate_main("tests/component_tests/web_server/web_server_auth_basic.yaml")
+    """Auth type basic on ESP32 uses plaintext credentials and does not warn."""
+    main_cpp = generate_main(
+        "tests/component_tests/web_server/web_server_auth_basic.yaml"
+    )
 
+    assert '->set_auth_username("admin");' in main_cpp
+    assert '->set_auth_password("password");' in main_cpp
+    assert "set_auth_basic_hash" not in main_cpp
     assert _has_define("USE_WEBSERVER_AUTH")
     assert not _has_define("USE_WEBSERVER_AUTH_DIGEST")
     assert _DEFAULT_CHANGE_WARNING not in caplog.text
 
 
+def test_web_server_auth_basic_esp8266_uses_precomputed_hash(
+    generate_main: Callable[[str], str],
+) -> None:
+    """Auth type basic on ESP8266 emits the precomputed base64 hash, not the credentials."""
+    main_cpp = generate_main(
+        "tests/component_tests/web_server/web_server_auth_basic_esp8266.yaml"
+    )
+
+    assert '->set_auth_basic_hash("YWRtaW46cGFzc3dvcmQ=");' in main_cpp
+    assert "set_auth_username" not in main_cpp
+    assert "set_auth_password" not in main_cpp
+    assert _has_define("USE_WEBSERVER_AUTH")
+    assert not _has_define("USE_WEBSERVER_AUTH_DIGEST")
+
+
+def test_web_server_auth_digest_esp8266_uses_plaintext_credentials(
+    generate_main: Callable[[str], str],
+) -> None:
+    """Auth type digest on ESP8266 uses plaintext credentials, not the basic hash."""
+    main_cpp = generate_main(
+        "tests/component_tests/web_server/web_server_auth_digest_esp8266.yaml"
+    )
+
+    assert '->set_auth_username("admin");' in main_cpp
+    assert '->set_auth_password("password");' in main_cpp
+    assert "set_auth_basic_hash" not in main_cpp
+    assert _has_define("USE_WEBSERVER_AUTH")
+    assert _has_define("USE_WEBSERVER_AUTH_DIGEST")
+
+
 def test_web_server_auth_explicit_digest(
     generate_main: Callable[[str], str],
     caplog: pytest.LogCaptureFixture,
diff --git a/tests/component_tests/web_server/web_server_auth_basic_esp8266.yaml b/tests/component_tests/web_server/web_server_auth_basic_esp8266.yaml
new file mode 100644
index 00000000000..79e0c0ccf53
--- /dev/null
+++ b/tests/component_tests/web_server/web_server_auth_basic_esp8266.yaml
@@ -0,0 +1,16 @@
+---
+esphome:
+  name: test
+
+esp8266:
+  board: esp01_1m
+
+wifi:
+  ssid: MySSID
+  password: password1
+
+web_server:
+  auth:
+    username: admin
+    password: password
+    type: basic
diff --git a/tests/component_tests/web_server/web_server_auth_digest_esp8266.yaml b/tests/component_tests/web_server/web_server_auth_digest_esp8266.yaml
new file mode 100644
index 00000000000..59565f8733e
--- /dev/null
+++ b/tests/component_tests/web_server/web_server_auth_digest_esp8266.yaml
@@ -0,0 +1,16 @@
+---
+esphome:
+  name: test
+
+esp8266:
+  board: esp01_1m
+
+wifi:
+  ssid: MySSID
+  password: password1
+
+web_server:
+  auth:
+    username: admin
+    password: password
+    type: digest
diff --git a/tests/components/web_server/test-basicauth.esp8266-ard.yaml b/tests/components/web_server/test-basicauth.esp8266-ard.yaml
new file mode 100644
index 00000000000..6a01180892a
--- /dev/null
+++ b/tests/components/web_server/test-basicauth.esp8266-ard.yaml
@@ -0,0 +1,8 @@
+packages:
+  web_server: !include common_v2.yaml
+
+web_server:
+  auth:
+    username: admin
+    password: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
+    type: basic
diff --git a/tests/components/web_server/test.rp2040-ard.yaml b/tests/components/web_server/test.rp2040-ard.yaml
index e4d50d77765..6a01180892a 100644
--- a/tests/components/web_server/test.rp2040-ard.yaml
+++ b/tests/components/web_server/test.rp2040-ard.yaml
@@ -4,5 +4,5 @@ packages:
 web_server:
   auth:
     username: admin
-    password: password
+    password: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
     type: basic