The 1931 Valentine, Texas, earthquake occurred in a region near the transition from the Southern Basin and Range/Rio Grande rift physiographic province to the Southern Great Plains province. Previous studies of this earthquake have presented focal mechanisms from first-motion data supporting either pure normal or strike-slip faulting with extension oriented in a southwest-northeast direction nearly perpendicular to the physiographic boundary between the provinces. This study presents the results of an inversion of regional and teleseismic body wave phases to determine earthquake source parameters. Results of the inversion suggest that the earthquake occurred at a depth of 10 km along a fault dipping 54° to the east with a strike of 10 to 20°, and a rake of −155°. The strike is similar to that of the southern segment of the nearby Mayfield fault, a fault that shows evidence for repeated movement during the Quaternary. Seismic and geodetic information suggest unilateral rupture toward the south during the earthquake. The T axis for the earthquake shows extension oriented in a northwest-southeast direction, a direction in agreement with the direction given by geologic stress indicators.