Revealing how diversity varies across the depth gradient is key to understanding the role of mesophotic coral ecosystems in the functioning of coral reefs. We examined how alpha and beta generic diversity of scleractinian coral assemblages vary across a wide depth gradient for coral reefs. We studied generic diversity patterns of scleractinian corals at sixteen sites in eight islands of three archipelagos in French Polynesia, as derived from the analysis of photo-quadrats, across the seafloor from shallow to lower mesophotic depths (6 to 120 m) and on a wide geographic scale. Our sampling considered quantitative coral cover to explore the patterns of alpha and beta components of diversity across depth and horizontal space. We show that in French Polynesia, mesophotic coral ecosystems host higher alpha and beta generic diversity than shallow reefs despite decreasing coral cover with depth. The variation of coral genus richness across the depth gradient is mainly driven by a mid-domain effect with a peak at 40 m depth. At the same time, we found that the turnover of coral genera across islands (i.e., spatial beta diversity) increased steadily along the depth gradient. Our findings report the first quantitative results of coral cover and diversity from mesophotic coral ecosystems in French Polynesia and also present one of the few existing studies to examine the broad breadth of the mesophotic depth gradient. We demonstrate that mesophotic depths can host unexpectedly high generic richness of scleractinian coral assemblages. At the same time, we showed that increasing depth increases the differences in generic diversity composition across islands, whereas shallow reefs are similar in between. While a single island could conserve shallow regional biodiversity, mesophotic depths containing the richest diversity require site-specific measures, suggesting that considering these mesophotic depths in conservation is necessary to maintain regional diversity.