The data on 87Sr/86Sr ratios of biologically available strontium in the environment, widely used in ancient population mobility studies, is scarce and scattered for most of the north-eastern Baltic Sea Region. This study includes new information on strontium isotope baselines in southern Finland. Archaeological samples of small mammals, mostly hare (Lepus timidus or europaeus) skeletal remains, were collected from southern Finland and measured for 87Sr/86Sr. Our goal is to develop a baseline for the local strontium isotope background level for comparison with archaeological human tooth enamel for information on past mobility. In general, the strontium isotopic baseline values in the region correlate with soil and bedrock geology, characterized by values of 0.730 or greater. As a case study, we present the analysis of individuals from the Ristimäki (‘Cross Hill’) inhumation cemetery and churchyard in Ravattula village, Kaarina municipality, southwestern Finland. The cemetery dates to Late Iron Age and Early Medieval times (12th–13th centuries) and contains the remains of the earliest known church in the country. The isotopic values from twelve graves indicate that the deceased were of local origin with the exception of two burials.