Cretaceous non-marine deposits are widespread in China and have been studied comprehensively. The Songliao Basin in northeast China is thought to be well suited for investigation of Cretaceous biostratigraphy. However, despite much research having been conducted in the basin, little is known about its Late Cretaceous biostratigraphy and paleoenvironment. Here, we establish a high-resolution biostratigraphy of the Late Cretaceous based on ostracods from borehole SK1(n) in the Songliao Basin, northeast China. As part of the present study, 45 species assigned to 20 genera have been recovered, with one new species (Ilyocypris bisulcatan. sp.) and five ostracod assemblages: theCypridea gunsulinensis–Mongolocypris magnaassemblage, which is marked by the first occurrence (F.O.) ofIlyocyprimorphawith nodes and spines; theIlyocyprimorpha–Limnocypridea sunliaonensis–Periacanthellaassemblage, which ranges from the F.O. ofIlyocyprimorphawith nodes and spines to the F.O. ofStrumosiasp.; theStrumosia inanditaassemblage from the F.O. ofStrumosiasp. to the lower occurrence (L.O.) ofStrumosia inandita; theTalicypridea amoena–Metacypris kaitunensis–Ziziphocypris simakoviassemblage from the F.O. ofMongolocypris apiculata(Cea) andTalicypridea amoenato the F.O. ofIlyocyprissp.; and theIlyocyprisassemblage from the F.O. ofIlyocyprissp. to the L.O. ofIlyocypris bisulcatan. sp.Moreover, the zonal fossilIlyocypris bisulcatan. sp. of Zone 5 is here described for the first time from the upper Mingshui Formation, and Paleocene charophyte genera includingNeocharaandGrovesicahrahave been found to coexist with the Zone 5 fauna. The age of theIlyocyprisAssemblage is assigned to the latest Maastrichtian to the earliest Danian.