Abstract Although the original concept of the peninsula effect was that fewer mammal species inhabited peninsulas than were found on the adjacent mainland, biogeographers have focused their attention on species richness gradients that bisect the long axes of peninsulas, instead of on the original concept. We evaluated the evidence for the peninsula effect, by comparing the richness of terrestrial mammal species on the Korean Peninsula with richness on mainland areas, and by investigating species richness gradients on 14 other peninsulas distributed globally. Our objectives were to: (1) demonstrate the existence of the peninsula effect, by comparing species richness on the Korean Peninsula with that on continental mainland areas of the same size and shape; and (2) test whether a declining gradient in species richness towards the tip represents a universal tendency associated with peninsulas, by investigating the species richness of 14 global peninsulas. We used the International Union for Conservation of Nature's spatial datasets of terrestrial mammals and the WorldClim climate dataset to create 1000 Korean Peninsula‐shaped polygons, and randomly placed them in continental areas. We recorded the number of mammalian species and mean annual temperature and precipitation for each polygon, and used these as variables in a linear regression model. We counted the number of mammal species along the central line of 14 peninsulas over 50000 km2 in size. Although the entire Korean Peninsula did not exhibit a significant peninsula effect (P = 0.1), South Korea did (P < 0.05) after controlling for temperature and precipitation. Fewer mammal species were found in South Korea than on similar continental areas. We modelled the general pattern of decline in species richness of mammals along 14 peninsulas (β  = ‐3.63, P < 0.001). In 10 peninsulas, a negative relationship existed between species richness and the distance from the mainland boundary; in four peninsulas, this relationship was positive. Isolated peninsulas without ‘stepping stone’ islands exhibited the strongest declines in mammalian species richness. Our results on the Korean Peninsula and declining patterns of species richness in 14 global peninsulas illustrate that the peninsula effect can be revealed with careful analyses, and may indicate effects on biogeography and biodiversity.