We present a least-squares solution for depth migration of the full reflected wavefield. The algorithm combines primary and high-order reflected energy and significantly enhances the image illumination and resolution compared to those of conventional migration. Least-squares full-wavefield migration (LS-FWM) directly computes the earth's reflectivity, thereby avoiding crosstalk noise often observed in imaging using high-order reflections. Iteratively solving an inversion problem is computationally intensive and can suffer from instability issues; however, we develop an efficient least-squares procedure by combining a fast and accurate one-way wave-equation propagator with an effective linear inversion solver. An advanced regularization method is also employed to stabilize the inversion by controlling the over-fitting problem. Successful applications to both synthetic and field data examples demonstrate that LS-FWM greatly improves the imaging illumination and resolution compared to conventional migration.