Geothermal gradients were estimated at the points of a grid with cell size of 50 km by 50 km in northwestern Colombia. Depths to the bottom of magnetic sources were assumed to represent Curie depths, and were estimated by means of statistical-spectral analysis of aeromagnetic data contained in square windows of 100 km by 100 km, 200 km by 200 km, and 300 km by 300 km. The centroid method and a variation of it that considers the fractal distribution of magnetization were applied. The modified centroid method provided better estimations of Curie depths, which range between 13 km and 47 km. Obtained Curie depths are comparable to those reported in other studies of regional character. The corresponding estimated geothermal gradients correlate quite well with estimations based on the bottom simulating reflector and bottom hole temperatures. Observed differences are small and can be accounted for by local heat flow variations due to shallow ground water flow, or recent sedimentation. Elaborated geothermal gradient and heat flow maps for northwestern Colombia are accurate and consistent with estimates of the thickness of the oceanic crust and continental cortical thicknesses, reported by previous authors. The maps presented in this study represent a contribution to the heat flow studies in northwestern South America.