A tire pressure detecting valve stem has a case, a body and a tire pressure detector. The case is rectangular and has an inner surface. The body is a valve stem and is integrally and transversely mounted on the case and has an inner section and a center pin. The inner section is integrally mounted in the case. The center pin is conductive, is coaxially mounted in the inner section and protrudes from the inner surface of the case. The tire pressure detector corresponds to and is mounted on the inner surface and is electronically connected to the center pin. Because the case is integrally and compactly mounted on the inner section of the body, the case will be mounted on and stay close to a rim when the body is mounted through the rim.