This dataset contains duplicate measurements of osmium isotope ratios and platinum group element concentrations for a clay layer from the Mokawaruppu section, Nemuro Group, Hokkaido, Japan. This clay layer was previously identified as the Cretaceous-Paleogene boundary layer by Saito et al. (1986). The analyses were performed to assess whether the clay layer represents an impact-derived boundary layer. Osmium isotope ratios and Os concentrations were measured by negative thermal ionization mass spectrometry (N-TIMS; TRITON, Thermo Fisher Scientific) and rhenium and other platinum group concentrations were measured by inductively coupled plasma quadrupole mass spectrometry (ICP-QMS; iCAP Q, ThermoFisher Scientific) at Japan Agency for Marine-Earth Science and Technology (JAMSTEC).