Tropical elevation gradients support highly diverse assemblages, but competing hypotheses suggest either peak species richness in lowland rainforests or at mid-elevations. We investigated scolytine beetles—phloem, ambrosia and seed-feeding beetles—along a tropical elevational gradient in Papua New Guinea. Highly standardised sampling from 200 to 3700 m above sea level (asl) identified areas of highest and lowest species richness, abundance and other biodiversity variables. Using passive flight intercept traps at eight elevations from 200 to 3500 m asl, we collected over 9600 specimens representing 215 species. Despite extensive sampling, species accumulation curves suggest that diversity was not fully exhausted. Scolytine species richness followed a unimodal distribution, peaking between 700 and 1200 m asl, supporting prior findings of highest diversity at low-to-mid elevations. Alternative models, such as a monotonous decrease from lowlands to higher elevations and a mid-elevation maximum, showed lesser fit to our data. Abundance is greatest at the lowest sites, driven by a few extremely abundant species. The turnover rate—beta diversity between elevation steps—is greatest between the highest elevations. Among dominant tribes—Dryocoetini, Xyleborini and Cryphalini—species richness peaked between 700 and 2200 m asl. Taxon-specific analyses revealed distinct patterns: Euwallacea spp. abundance uniformly declined with elevation, while other genera were driven by dominant species at different elevations. Coccotrypes and phloem-feeding Cryphalus have undergone evolutionary radiations in New Guinea, with many species still undescribed. Species not yet known to science are most likely to be found at lower and middle elevations, where overall diversity is highest.