diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5bfa4e78dde..bbf41952c26 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,2 +1,3 @@
+- Added a deprecation warning for functions.config() to stderr on deploy and all functions:config commands. (#8808)
 - Added analytics to track runtime config usage in functions deployments (#8870).
 - Fixed issue where `__name__` fields with DESCENDING order were incorrectly filtered from index listings, causing duplicate index issues (#7629) and deployment conflicts (#8859). The fix now preserves `__name__` fields with explicit DESCENDING order while filtering out implicit ASCENDING `__name__` fields.
diff --git a/src/commands/functions-config-clone.ts b/src/commands/functions-config-clone.ts
index 5fe6cc8d614..12b0d2c129d 100644
--- a/src/commands/functions-config-clone.ts
+++ b/src/commands/functions-config-clone.ts
@@ -8,6 +8,7 @@ import { requirePermissions } from "../requirePermissions";
 import * as functionsConfig from "../functionsConfig";
 import { functionsConfigClone } from "../functionsConfigClone";
 import * as utils from "../utils";
+import { logFunctionsConfigDeprecationWarning } from "../functions/deprecationWarnings";
 
 export const command = new Command("functions:config:clone")
   .description("clone environment config from another project")
@@ -56,4 +57,5 @@ export const command = new Command("functions:config:clone")
         "firebase deploy --only functions",
       )}\n`,
     );
+    logFunctionsConfigDeprecationWarning();
   });
diff --git a/src/commands/functions-config-get.ts b/src/commands/functions-config-get.ts
index 6367b0c7c9f..3827b80d2cf 100644
--- a/src/commands/functions-config-get.ts
+++ b/src/commands/functions-config-get.ts
@@ -6,6 +6,7 @@ import { logger } from "../logger";
 import { needProjectId } from "../projectUtils";
 import { requirePermissions } from "../requirePermissions";
 import * as functionsConfig from "../functionsConfig";
+import { logFunctionsConfigDeprecationWarning } from "../functions/deprecationWarnings";
 
 async function materialize(projectId: string, path?: string): Promise<any> {
   if (path === undefined) {
@@ -31,5 +32,6 @@ export const command = new Command("functions:config:get [path]")
   .action(async (path, options) => {
     const result = await materialize(needProjectId(options), path);
     logger.info(JSON.stringify(result, null, 2));
+    logFunctionsConfigDeprecationWarning();
     return result;
   });
diff --git a/src/commands/functions-config-set.ts b/src/commands/functions-config-set.ts
index f29e37c0bbe..6ab9aa1f67a 100644
--- a/src/commands/functions-config-set.ts
+++ b/src/commands/functions-config-set.ts
@@ -7,6 +7,7 @@ import { needProjectId } from "../projectUtils";
 import { requirePermissions } from "../requirePermissions";
 import * as functionsConfig from "../functionsConfig";
 import * as utils from "../utils";
+import { logFunctionsConfigDeprecationWarning } from "../functions/deprecationWarnings";
 
 export const command = new Command("functions:config:set [values...]")
   .description("set environment config with key=value syntax")
@@ -49,4 +50,5 @@ export const command = new Command("functions:config:set [values...]")
         "firebase deploy --only functions",
       )}\n`,
     );
+    logFunctionsConfigDeprecationWarning();
   });
diff --git a/src/commands/functions-config-unset.ts b/src/commands/functions-config-unset.ts
index 97f989597e2..4b326876236 100644
--- a/src/commands/functions-config-unset.ts
+++ b/src/commands/functions-config-unset.ts
@@ -8,6 +8,7 @@ import * as functionsConfig from "../functionsConfig";
 import * as runtimeconfig from "../gcp/runtimeconfig";
 import * as utils from "../utils";
 import { FirebaseError } from "../error";
+import { logFunctionsConfigDeprecationWarning } from "../functions/deprecationWarnings";
 
 export const command = new Command("functions:config:unset [keys...]")
   .description("unset environment config at the specified path(s)")
@@ -44,4 +45,5 @@ export const command = new Command("functions:config:unset [keys...]")
         "firebase deploy --only functions",
       )}\n`,
     );
+    logFunctionsConfigDeprecationWarning();
   });
diff --git a/src/deploy/functions/prepareFunctionsUpload.ts b/src/deploy/functions/prepareFunctionsUpload.ts
index 175b8478252..f5fd2e94581 100644
--- a/src/deploy/functions/prepareFunctionsUpload.ts
+++ b/src/deploy/functions/prepareFunctionsUpload.ts
@@ -13,6 +13,7 @@ import * as functionsConfig from "../../functionsConfig";
 import * as utils from "../../utils";
 import * as fsAsync from "../../fsAsync";
 import * as projectConfig from "../../functions/projectConfig";
+import { logFunctionsConfigDeprecationWarning } from "../../functions/deprecationWarnings";
 
 const CONFIG_DEST_FILE = ".runtimeconfig.json";
 
@@ -99,6 +100,9 @@ async function packageSource(
         name: CONFIG_DEST_FILE,
         mode: 420 /* 0o644 */,
       });
+
+      // Log deprecation warning when runtime config is being packaged
+      logFunctionsConfigDeprecationWarning();
     }
     await pipeAsync(archive, fileStream);
   } catch (err: any) {
diff --git a/src/experiments.ts b/src/experiments.ts
index 615c7459032..6027b9d5d3a 100644
--- a/src/experiments.ts
+++ b/src/experiments.ts
@@ -57,6 +57,14 @@ export const ALL_EXPERIMENTS = experiments({
       "of how that image was created.",
     public: true,
   },
+  dangerouslyAllowFunctionsConfig: {
+    shortDescription: "Allows the use of deprecated functions.config() API",
+    fullDescription:
+      "The functions.config() API is deprecated and will be removed on December 31, 2025. " +
+      "This experiment allows continued use of the API during the migration period.",
+    default: true,
+    public: true,
+  },
 
   // Emulator experiments
   emulatoruisnapshot: {
diff --git a/src/functions/deprecationWarnings.ts b/src/functions/deprecationWarnings.ts
new file mode 100644
index 00000000000..96b98f65c63
--- /dev/null
+++ b/src/functions/deprecationWarnings.ts
@@ -0,0 +1,22 @@
+import { logWarningToStderr } from "../utils";
+
+const FUNCTIONS_CONFIG_DEPRECATION_MESSAGE = `DEPRECATION NOTICE: Action required to deploy after Dec 31, 2025
+  
+  functions.config() API is deprecated.
+  Cloud Runtime Configuration API, the Google Cloud service used to store function configuration data, will be shut down on December 31, 2025. As a result, you must migrate away from using functions.config() to continue deploying your functions after December 31, 2025.
+  
+  What this means for you:
+  
+  - The Firebase CLI commands for managing this configuration (functions:config:set, get, unset, clone, and export) are deprecated. These commands no longer work after December 31, 2025.
+  - firebase deploy command will fail for functions that use the legacy functions.config() API after December 31, 2025.
+  
+  Existing deployments will continue to work with their current configuration.
+  
+  See your migration options at: https://firebase.google.com/docs/functions/config-env#migrate-to-dotenv`;
+
+/**
+ * Logs a deprecation warning for functions.config() usage
+ */
+export function logFunctionsConfigDeprecationWarning(): void {
+  logWarningToStderr(FUNCTIONS_CONFIG_DEPRECATION_MESSAGE);
+}
diff --git a/src/utils.ts b/src/utils.ts
index c5393b52da0..275cb313034 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -198,6 +198,14 @@ export function logWarning(
   logger[type](clc.yellow(clc.bold(`${WARNING_CHAR} `)), message, data);
 }
 
+/**
+ * Log a warning statement to stderr, regardless of logger configuration.
+ */
+export function logWarningToStderr(message: string): void {
+  const prefix = clc.bold(`${WARNING_CHAR} `);
+  process.stderr.write(clc.yellow(prefix + message) + "\n");
+}
+
 /**
  * Log an info statement with a gray bullet at the start of the line.
  */