The Foremost Formation of southern Alberta is the basal member of the Campanian Belly River Group, and has been understudied relative to the overlying Oldman and Dinosaur Park formations of this group. Examined outcrop of the upper Foremost Formation preserves coastal barrier island/lagoonal environments that transition upward into more inland coastal/ fluvial plain environments during the Claggett marine regression of the Western Interior Seaway. A low abundance, low diversity agglutinated benthic foraminiferal fauna, together with whole rock geochemical proxies, suggests commonly stressed oxygen and salinity conditions in marginal marine environments. Vertebrate microsite faunas display a primarily brackish water environmental influence, with an increase in diversity upsection in conjunction with increased terrestrial deposition. Hadrosaur remains suggest the previously unknown presence of kritosaurs and potentially a basal hadrosauroid, suggesting that the Foremost Formation dinosaur fauna was similarly diverse compared to faunas of well-characterized Campanian formations in North America.