A stable optical resonator has been built using a 30 μm-wide, metal-coated microcantilever as one mirror. The second mirror was a 12.7 mm-diameter concave dielectric mirror. By positioning the two mirrors 75mm apart in a near-hemispherical configuration, a Fabry-Pérot cavity with a finesse equal to 55 was achieved. The finesse was limited by the optical loss in the cantilever’s metal coating; diffraction losses from the small mirror were negligible. The cavity achieved passive laser cooling of the cantilever’s Brownian motion.