diff --git a/src/channels/plugins/onboarding/signal.test.ts b/src/channels/plugins/onboarding/signal.test.ts
new file mode 100644
index 00000000000..2c055e2ec94
--- /dev/null
+++ b/src/channels/plugins/onboarding/signal.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, it } from "vitest";
+
+import { normalizeSignalAccountInput } from "./signal.js";
+
+describe("normalizeSignalAccountInput", () => {
+  it("accepts already normalized numbers", () => {
+    expect(normalizeSignalAccountInput("+15555550123")).toBe("+15555550123");
+  });
+
+  it("normalizes formatted input", () => {
+    expect(normalizeSignalAccountInput("  +1 (555) 000-1234 ")).toBe("+15550001234");
+  });
+
+  it("rejects empty input", () => {
+    expect(normalizeSignalAccountInput("   ")).toBeNull();
+  });
+
+  it("rejects non-numeric input", () => {
+    expect(normalizeSignalAccountInput("ok")).toBeNull();
+    expect(normalizeSignalAccountInput("++--")).toBeNull();
+  });
+
+  it("rejects numbers that are too short or too long", () => {
+    expect(normalizeSignalAccountInput("+1234")).toBeNull();
+    expect(normalizeSignalAccountInput("+1234567890123456")).toBeNull();
+  });
+});
diff --git a/src/channels/plugins/onboarding/signal.ts b/src/channels/plugins/onboarding/signal.ts
index 3f5b969e5d5..168efec03fa 100644
--- a/src/channels/plugins/onboarding/signal.ts
+++ b/src/channels/plugins/onboarding/signal.ts
@@ -16,6 +16,23 @@ import { normalizeE164 } from "../../../utils.js";
 import { addWildcardAllowFrom, promptAccountId } from "./helpers.js";
 
 const channel = "signal" as const;
+const MIN_E164_DIGITS = 5;
+const MAX_E164_DIGITS = 15;
+const INVALID_SIGNAL_ACCOUNT_ERROR =
+  "Invalid E.164 phone number (must start with + and country code, e.g. +15555550123)";
+
+export function normalizeSignalAccountInput(value: string | null | undefined): string | null {
+  const trimmed = value?.trim();
+  if (!trimmed) {
+    return null;
+  }
+  const normalized = normalizeE164(trimmed);
+  const digits = normalized.slice(1);
+  if (digits.length < MIN_E164_DIGITS || digits.length > MAX_E164_DIGITS) {
+    return null;
+  }
+  return normalized;
+}
 
 function setSignalDmPolicy(cfg: OpenClawConfig, dmPolicy: DmPolicy) {
   const allowFrom =
@@ -243,22 +260,34 @@ export const signalOnboardingAdapter: ChannelOnboardingAdapter = {
 
     let account = accountConfig.account ?? "";
     if (account) {
-      const keep = await prompter.confirm({
-        message: `Signal account set (${account}). Keep it?`,
-        initialValue: true,
-      });
-      if (!keep) {
+      const normalizedExisting = normalizeSignalAccountInput(account);
+      if (!normalizedExisting) {
+        await prompter.note(
+          "Existing Signal account isn't a valid E.164 number. Please enter it again.",
+          "Signal",
+        );
         account = "";
+      } else {
+        account = normalizedExisting;
+        const keep = await prompter.confirm({
+          message: `Signal account set (${account}). Keep it?`,
+          initialValue: true,
+        });
+        if (!keep) account = "";
       }
     }
 
     if (!account) {
-      account = String(
+      const rawAccount = String(
         await prompter.text({
           message: "Signal bot number (E.164)",
-          validate: (value) => (value?.trim() ? undefined : "Required"),
+          validate: (value) =>
+            normalizeSignalAccountInput(String(value ?? ""))
+              ? undefined
+              : INVALID_SIGNAL_ACCOUNT_ERROR,
         }),
-      ).trim();
+      );
+      account = normalizeSignalAccountInput(rawAccount) ?? "";
     }
 
     if (account) {