ABSTRACT: Sympatric organisms can avoid competition by occupying different ecological niches, a mechanism known as niche partitioning. In Iceland, white-beaked dolphins Lagenorhynchus albirostris and harbour porpoises Phocoena phocoena are the most common small cetaceans observed but their trophic ecology remains understudied, including the potential for resource competition. In this study, we measured δ¹⁵N and δ¹³C values from white-beaked dolphin (n = 28) and harbour porpoise (n = 29) muscle samples collected over a 33 yr period (1987-2019) to compare isotopic niche width and overlap of both species, as well as diet composition. We also tested for within-species effects of sex, age class, body length, year of sampling and origin (bycatch vs. stranding) on the δ¹³C and δ¹⁵N values. Intra-species variations included differences between stranded and bycaught white-beaked dolphins and ontogenetic variations in both species, but further studies are necessary to investigate the factors that might explain these results. Inter-species comparisons showed that harbour porpoises and white-beaked dolphins had non-overlapping isotopic niches, with harbour porpoises exhibiting smaller niche widths (corrected standard ellipse area, SEA_c: 1.25 vs. 2.13‰²). While white-beaked dolphins had a broader diet, composed primarily of gadoids but with contributions of other fish species, harbour porpoises fed almost exclusively on capelin Mallotus villosus, suggesting niche segregation and width is largely maintained by targeting different prey resources. Both species also showed no long-term changes in trophic ecology, despite recent ecosystem changes in this region, possibly a result of adapting to spatial changes in prey distribution or shifts to other prey at similar trophic levels.