Multiple burial in medieval burial grounds are often interpreted as a result of disease, but it is difficult to test such hypotheses, as most acute infectious diseases leave no visible evidence on skeletal material. Scientific dating can potentially associate multiple burials with historically documented epidemics, but the precision required to exclude alternative explanations would normally be attainable only by dendrochronology. Here, we argue that by combining archaeological, osteological and paleodiet research in a Bayesian framework, we can exploit differences in dietary reservoir effects to refine the dates of multiple burials, and potentially date such events to within a range of <20 years. We present new radiocarbon (14C) and stable isotope (δ13C, δ15N) results from a medieval multiple grave at St Alban’s Odense, on the island of Funen in central Denmark. We show the ca. 150-yr spread in 14C ages of the five juveniles is compatible with differences in the amount of fish they consumed. Our chronological model, which combines marine reservoir effect correction with calendar age offsets based on osteological evidence, dates the multiple burial to cal AD 1425–1445 (95% probability), an interval in which two plague epidemics took place in Denmark.