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Variation in dorsal fin morphology was assessed in five common bottlenose dolphin populations from the Southeast Pacific. We hypothesized that habitat specialization between coastal and offshore ecotypes would lead to differences in dorsal fin morphology. Photographs and direct measurements of dorsal fins were used to calculate three indexes: height/length base (h/b), width at half height/length base (a/b), and overhang of the dorsal fin tip/length base (falcateness) (s/b). The sample included 163 individuals (129 coastal and 34 offshore) from Ecuador, 60 individuals (nine coastal and 51 offshore) from Peru, and 25 individuals of an inshore community occurring in north-central Chile (Pod-R). Ontogenetic variation was found in coastal dolphins from Ecuador, where sex and age classes were best represented. A statistically significant difference was found in the a/b index between coastal specimens from Ecuador and Peru and among the three offshore groups. When offshore and Pod-R data were pooled and compared with data from coastal specimens from Ecuador and Peru, a significant difference was found in the s/b index. Overall, dorsal fins of offshore dolphins are relatively higher than fins of coastal individuals. However, the most consistent difference between ecotypes was the strong falcateness (high s/b) in offshore forms versus a more triangular shape (low s/b) in coastal forms. We propose that dorsal fin falcateness is a reliable criterion to visually distinguish between bottlenose dolphin ecotypes in this region. Proper identification in the field greatly facilitates research and helps focus management needs of the different bottlenose dolphin populations.