We have analyzed a series of 22 groundfish surveys conducted between 1989 and 1999 on the Portuguese shelf and upper slope (97 sampling station grid, 36−710 m) in the summer, autumn, and winter, to address the hypotheses of latitudinal and depth trends in species richness (number of species) and whether these patterns changed over time and among seasons. Richness was modelled by means of a Generalized Additive Model (GAM) with significant covariates being latitude and longitude, in a joint-spatial smooth, and depth, chronological time, and season. A decrease in richness was detected along the continental margin, when moving higher in latitude, with lowest richness being found in the shallow northern area. Thus, although our study spanned only about 5 degrees in latitude, we found no exception to the equator-pole latitudinal decrease in richness commonly acknowledged by ecologists. There is no common agreement on the way marine fish species richness varies with depth, yet our findings were consistent for the entire eleven-year period: richness decreases slightly with depth, from the coastline to the shelf break (200−300 m), steadily increasing afterwards down the slope. The patterns of change in latitude and depth persisted in time and among seasons but their magnitude changed. Richness was higher in the winter compared to the other seasons.