In this letter, a torsional optomechanical resonator for torque sensing and torsional mechanical frequency mixing is experimentally demonstrated. The torsional mechanical resonator is embedded into a split optical racetrack resonator, which provides high sensitivity in measuring torsional mechanical motion. Using this high sensitivity, torsional mechanical frequency mixing is observed without regenerative mechanical motion. The displacement noise floor of the torsional mechanical resonator is 50 fm/Hz0.5, which demonstrates a resonant torque sensitivity of 3.58 10 21Nm/Hz0.5. This demonstration will benefit potential applications for on-chip RF signal modulation using optical mechanical resonators.