A tire-mounted sensor is to be mounted to an inner wall surface of a tire, and includes a sensor device having a vibration detection element to detect vibration applied to the tire, a circuit board having the vibration detection element thereon, an antenna attached to the circuit board, and an accommodation structure accommodating the vibration detection element, the circuit board, and the antenna. At least a part of the accommodation structure is made of a flexible material to attenuate the vibration applied to the tire. The tire-mounted sensor further includes a vibration transmission member configured to transmit the vibration applied to the tire to the vibration detection element.