The unique pattern of small tubercles on the leading edge of the dorsal fins of harbor porpoises (Phocoena phocoena) has been widely noted in the literature, though their structure or function has never been conclusively identified. We examined external morphology and microanatomy of the tubercles for further understanding of the nature of the tubercles. Measurements were taken of height and peak-to-peak distance of the tubercles using scaled photographs. Mean tubercle height was standardized as a percentage of the dorsal fin height and ranged from 0.63 to 0.87%. Mean peak-to-peak distance ranged from 4.2 ± 2.0 to 5.6 ± 2.0 mm. The microstructure analysis of the dorsal fin leading edge, trailing edge and tubercles revealed an epidermal thickness of 0.7–2.7 mm with the thickest epidermis at the tubercular apex. The epidermis contained three distinct strata (=layers), including the stratum corneum, spinosum, and basale. The stratum corneum was significantly thickened in tubercles, over four times thicker than in the leading or trailing edge of the fin. The stratum spinosum, composed of lipokeratinocytes and lamellar oil bodies, was significantly thinner in the trailing edge than in the other two sites. There was no significant difference in the stratum basale among the three sites. Volume fraction of lipokeratinocytes was significantly higher at the sides of the leading edge and the apex of the tubercles, while volume fraction of lamellar oil bodies was significantly lower at the apex of the tubercles. Though the function of the tubercles is unknown, their position, hardened structure and increased epidermal stratum corneum suggest that they may have hydrodynamic importance.