An adapter assembly for attaching a tire pressure monitor onto an inflation valve which incorporates a distal end flange. An adapter is included in the assembly that consists of a collar having a configuration sized to fit over and be internally retained by the valve distal end flange. An O-ring is positioned on the flange contiguously engaging the collar. A tire pressure monitoring cap is threadably attached to the collar which is disposed above and compresses the O-ring between the cap, collar and valve end flange. The adapter permits the tire pressure monitor to be attached, for use with a Tire Pressure Monitoring System (TPMS) and also prevents minor air leaks. A fourth embodiment incorporates an outward-extending integral arm formed with the collar for mounting the TPMS sensor on the drop center of the wheel.