Biodiversity patterns along elevational gradients are generally characterised by monotonic decreases or mid-elevational peaks in species richness, while elevational zones may be characterised by distinct assemblages, or higher zones may be subsets of lowland assemblages. Elevational gradients in diversity have been less studied in the Afrotropical region. This study documents ant diversity patterns in three forest types associated with the tropical mountains of Udzungwa; we hypothesise that: (1) ant diversity and activity will show a monotonic decrease from mid-elevation with increasing elevation and (2) that forests associated with different elevations will have a distinct ant assemblage. Pitfall traps were deployed at three targeted elevations (650–800, 800–1400, and 1400–1500 m a.s.l.). Ant species richness declined with increasing elevation from 650 m a.s.l. and formed three elevational assemblages with lower elevation forests having almost twice as many species as sub-montane forests and three times as many as that of the montane forests. In contrast, overall ant activity peaked at 800–1400 m a.s.l. The ant assemblages associated with the lower elevation forest were very distinct, while assemblages associated with the sub-montane and montane forests shared species. Our study reveals valuable and relevant information for biodiversity monitoring and conservation planning as the species associated with each forest type may be used as indicator species for assessing biodiversity responses to climate change and anthropogenic activities on these mountains.