diff --git a/packages/arg-parser/src/arg-mapper.spec.ts b/packages/arg-parser/src/arg-mapper.spec.ts
index 7aa593b55c..342ad9d245 100644
--- a/packages/arg-parser/src/arg-mapper.spec.ts
+++ b/packages/arg-parser/src/arg-mapper.spec.ts
@@ -77,8 +77,8 @@ describe('arg-mapper.mapCliToDriver', () => {
 
   context('when cli args have password', () => {
     it('maps to auth object', () => {
-      expect(optionsTest({ password: 'aphextwin' })).to.deep.equal({
-        cs: 'mongodb://:aphextwin@localhost/'
+      expect(optionsTest({ password: 'aphextwin', username: 'x' })).to.deep.equal({
+        cs: 'mongodb://x:aphextwin@localhost/'
       });
     });
   });
@@ -340,4 +340,21 @@ describe('arg-mapper.mapCliToDriver', () => {
       });
     });
   });
+
+  context('when password is provided without username', () => {
+    const cliOptions: CliOptions = { password: '1234' };
+
+    it('throws a helpful error', () => {
+      expect(() => optionsTest(cliOptions)).to.throw(
+        "[COMMON-10001] Invalid connection information: Password specified but no username provided (did you mean '--port' instead of '-p'?)");
+    });
+  });
+  context('when password is provided without username and --port is already specified', () => {
+    const cliOptions: CliOptions = { password: '1234', port: '12345' };
+
+    it('throws a helpful error', () => {
+      expect(() => optionsTest(cliOptions)).to.throw(
+        '[COMMON-10001] Invalid connection information: Password specified but no username provided');
+    });
+  });
 });
diff --git a/packages/arg-parser/src/arg-mapper.ts b/packages/arg-parser/src/arg-mapper.ts
index 463987dcbf..fc2b95908b 100644
--- a/packages/arg-parser/src/arg-mapper.ts
+++ b/packages/arg-parser/src/arg-mapper.ts
@@ -1,3 +1,4 @@
+import { CommonErrors, MongoshInvalidInputError } from '@mongosh/errors';
 import type { CliOptions, ConnectionInfo } from './';
 import type { DevtoolsConnectOptions } from '@mongodb-js/devtools-connect';
 import { ConnectionString, CommaAndColonSeparatedRecord } from 'mongodb-connection-string-url';
@@ -152,6 +153,8 @@ export function mapCliToDriver(options: Readonly<CliOptions>, i: Readonly<Connec
     }
   }
 
+  validateConnectionInfoAfterArgMapping(i, options);
+
   return i;
 }
 
@@ -167,4 +170,24 @@ function mapGSSAPIHostnameCanonicalization(value: string): AuthMechanismProps['C
   return value as AuthMechanismProps['CANONICALIZE_HOST_NAME'];
 }
 
+function validateConnectionInfoAfterArgMapping(info: ConnectionInfo, originalOptions: CliOptions): void {
+  if (!info.connectionString) {
+    return;
+  }
+  // Provide a better error message for some cases in which applying
+  // command line options to the connection string can make it invalid.
+  const connectionString = new ConnectionString(info.connectionString, { looseValidation: true });
+  if (connectionString.password && !connectionString.username) {
+    let text = 'Invalid connection information: Password specified but no username provided';
+    if (originalOptions.password && !originalOptions.port) {
+      text += " (did you mean '--port' instead of '-p'?)";
+    }
+    throw new MongoshInvalidInputError(text, CommonErrors.InvalidArgument);
+  }
+  // Just make sure the result ultimately parses with strict validation.
+  // eslint-disable-next-line no-new
+  new ConnectionString(info.connectionString, { looseValidation: false });
+}
+
+
 export default mapCliToDriver;