Light trapping in optical cavities has many applications in optical telecommunications, biomedical optics, atomic studies, and chemical analysis. Efficient optical coupling in these cavities is an important engineering problem that affects greatly the cavity performance. Reported in-plane external fiber Fabry–Perot cavities in the literature are based on flat micromachined mirrors. In this case, the diffraction loss in the cavity is usually overcome by using an expensive-lensed fiber or by inserting a coated lens in the cavity leading to a long cavity with a small free spectral range. In this work, we report a Fabry–Perot cavity formed by a multilayer-coated cleaved-surface single-mode fiber inserted into a groove while facing a three-dimensional concave micromirror; both are fabricated by silicon micromachining. The light is trapped inside the cavity while propagating in-plane of the wafer substrate. Theoretical modeling is carried out, taking into account the effect of asymmetry in the mirror radii of curvature resulting from the micromachining process. A cavity is formed using a concave mirror with 200 and 100 μm in-plane and out-of-plane radii curvature, respectively. The presented cavity has a measured line width of 0.45 nm around 1330 nm showing a quality factor of about 3000, which resembles a one order of magnitude improvement over a flat-mirror cavity.