On 12 January 2010, a Mw7.0 earthquake struck the Port-au-Prince region of Haiti. The disaster killed more than 200,000 people and caused an estimated $8 billion in damages, about 100% of the country's gross domestic product. The earthquake was initially thought to have ruptured the Enriquillo-Plantain Garden fault of the southern peninsula of Haiti, which is one of two main strike-slip faults inferred to accommodate the 2cmyr-1 relative motion between the Caribbean and North American plates. Here we use global positioning system and radar interferometry measurements of ground motion to show that the earthquake involved a combination of horizontal and contractional slip, causing transpressional motion. This result is consistent with the long-term pattern of strain accumulation in Hispaniola. The unexpected contractional deformation caused by the earthquake and by the pattern of strain accumulation indicates present activity on faults other than the Enriquillo-Plantain Garden fault. We show that the earthquake instead ruptured an unmapped north-dipping fault, called the Léogâne fault. The Léogâne fault lies subparallel to-but is different from-the Enriquillo-Plantain Garden fault. We suggest that the 2010 earthquake may have activated the southernmost front of the Haitian fold-and-thrust belt as it abuts against the Enriquillo-Plantain Garden fault. As the Enriquillo-Plantain Garden fault did not release any significant accumulated elastic strain, it remains a significant seismic threat for Haiti and for Port-au-Prince in particular.