Abstract The magnetic domain structure of a Ni 49.9Mn 28.3Ga 21.8 ferromagnetic shape memory alloy has been investigated by in situ Lorentz TEM. Field-induced changes in the magnetic domain wall structure were recorded over a field range of [−500, +300] Oe. Inside a martensite twin variant, the observed domain structure was either an alternating 180° wall pattern or a maze-like pattern, depending on the relative orientation of the magnetic easy axis and the in-plane applied field. In twin variants with an in-plane easy axis, significant domain wall movement was observed at moderate applied fields, in agreement with an existing magneto-mechanical model. 180° domain walls were found to be pinned by anti-phase boundaries (APBs). The maze-like domain structure was stable under applied fields below about ±100 Oe; at higher fields, the walls became aligned with the applied field. Domain walls also remained strongly pinned at twin boundaries up to applied fields of around 400 Oe. Interestingly, depinning of walls from twin boundaries occurs at field values that are significantly lower than those required to induce motion of the structural twins.