Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers(1). The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate(2,3) and fast-spreading centres(4,5), but it has been difficult to image such magma chambers beneath slow-spreading centres(6,7), owing to rough seafloor topography and associated seafloor scattering(7,8). In the absence of any images of magma chambers(6) or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3 - 4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.