The widespread coastal hermit crab Pagurus longicarpus relies on empty gastropod shells for shelter. At low tide, these hermit crabs often become stranded in tide pools, where changes in temperature and salinity can occur rapidly. We tested how changes in temperature and salinity affected the sizes of the shells chosen by hermit crabs. Increasing the seawater temperature from 22 °C to 32 °C had a significant effect () on the size of shells chosen, as did lowering the temperature to 16 °C. Reducing the salinity from 30 ppt to 20 ppt or raising it to 40 ppt did not affect shell choice significantly, but lowering the salinity from 30 ppt to 15 ppt did. This study builds on previous studies of shell size preference of hermit crabs by highlighting the importance of taking temperature and salinity into account when assigning shells of appropriate size in laboratory studies. These results also suggest that climate change may alter the sizes of shells that hermit crabs of the species P. longicarpus will seek in the field. Additional studies will be required to evaluate the potential impacts of such shifts on hermit crab survival and reproduction.