Two densely sampled marine refraction lines were shot in northern Baffin Bay on the shelves of Devon and Ellesmere islands (North American plate) and Greenland (Greenland plate). A total of 11 ocean-bottom seismometers recorded the airgun signals. The processed data were analyzed by the use of ray tracing and amplitude modelling. Two-dimensional models were derived that reproduce the characteristics of the observed data. A 5 km deep sedimentary basin was identified on the south end of line 3. On both lines the crustal velocity has a range of 5.7–6.6 km/s. Midway along the line on the shelf of Devon and Ellesmere islands, the Moho shallows abruptly northward from 27 to 20 km. The thinned crust is not overlain by a sedimentary basin to compensate for the elevated Moho, suggesting this is not an extensional feature. The thickness of the crust adjacent to northwest Greenland increases from south (22 km) to north (37 km). The thickening occurs in two stages: a sharp increase in the depth to Moho northwest of the sedimentary basin followed by a gradual deepening to the end of the line. The thin crust on the shelf of Ellesmere Island is located adjacent to the thick crust of Greenland. Plate reconstructions based on regional magnetic anomalies and transform faults indicate that Greenland is a separate plate. The crustal structure revealed by seismic refraction and reflection profiles and the variations in the depth to Moho are consistent with the plate boundary occurring between the refraction lines.