Interferometric synthetic aperture radar (InSAR) measurements, field observations and elastic modelling of the 2009–2010 Karonga (northern Malawi) earthquake swarm reveal widespread coseismic and localized post-seismic deformation. In a period of about 1.5 months starting on November 5, 29 M ≥ 4 earthquakes struck the region, culminating in an Mw 6 peak event on December 19. The next few months were characterized by significant localized deformation with a very low seismic moment release. We find a very good agreement between InSAR and field observations of surface ruptures. Our best fitted coseismic models indicate dip-slip displacements on a fault dipping 40° to the southwest with maximum slip of about 120 cm at 3–5 km depth. Fault activity continued until 2010 August as shallow aseismic afterslip mostly above the maximum coseismic slip patches. Although the swarm occurred within a coastal plain covered by porous Quaternary sediments and a high water table, the effect of poroelastic relaxation on the post-seismic deformation was found to be negligibly small. In contrast with other recent earthquake swarms along the East African Rift, the Karonga swarm shows no evidence for dike intrusion.