<p>Human mortality rates are known to be related to ambient temperature and are expected to be affected by climate change. We applied a distributed lag non-linear model (DLNM) to determine regional temperature-mortality relationships in 19 out of 23 Finnish wellbeing service counties, based on daily death and temperature data from the period 2000–2017. We estimated future heat- and cold-related mortality during this century using climate projections assuming moderate (SSP2-4.5) and very high (SSP5-8.5) greenhouse gas emissions. For Helsinki, we also investigated how population projections and adaptation measures might affect temperature-related mortality rates. Our results project a decline in deaths attributable to cold and an increase in deaths attributable to heat during this century over most counties, though with some regional heterogeneity. In 12 out of 19 counties consistent increasing trends in future heat-related mortality were demonstrated with the highest increases in counties in southern and eastern Finland. Also, in 14 out of 19 counties there is consistent decrease in cold-related deaths. Future projections that also account for potential population growth in Helsinki by the end of the century, increase estimates of both heat- and cold-related deaths. However, the results should be interpreted with caution due to substantial uncertainty. There is currently a lack of long-term population projections and associated uncertainties for different regions and age groups in Finland. Further studies on temperature-related mortality should be based on longer time series with a wider range of recent observed temperature extremes as well as more refined sociodemographic predictor variables.</p>