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 dielectricmirror. 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.