The carbon stable isotope composition (δ¹³C) of tooth enamel in mosasaurid squamates reflects aspects of their diet and diving behaviour. Here we present new δ¹³C data for such marine squamates from the Maastrichtian of Denmark and compare these with results obtained in previous studies from the lower-latitude type area of the Maastrichtian Stage (latest Cretaceous; 72.1–66.0 Ma) in the south-east Netherlands and north-east Belgium. For the Danish samples, there is a weak correlation between mosasaur body size and δ¹³C values, with larger-sized taxa having lower δ¹³C values, comparable to what has previously been observed for mosasaurs from the Maastrichtian type area.