Assessments of global Pliocene climate using conventional proxies indicate prevalent warmth at mid- to high latitudes. How this climate change was manifested on a regional basis and on annual timescales remains poorly understood. Oxygen isotope ratios (δ18O) of fossil Mercenaria spp. (bivalve) shells enable reconstruction of growth temperature to estimate sea surface temperature (SST) at seasonal resolution. Comparing growth characteristics of fossil and modern populations potentially provides independent SST constraints. Fossil shells were selected from the Rushmere Member of the Yorktown Formation (Pliocene, early Piacenzian, ∼3.3–3.0 Ma, warm interval) and the Chowan River Formation (Pleistocene, Gelasian, 2.4–1.8 Ma, subsequent cooling) of the U.S. Mid-Atlantic Coastal Plain (MACP). The coldest winter temperatures recorded in the Yorktown Formation shells averaged 17 ± 2 °C and the warmest summer temperatures averaged 25 ± 2 °C. Chowan River Formation shells recorded the coldest winter temperatures averaging 12 ± 2 °C and the warmest summer temperatures averaging 21 ± 2 °C. Modern winter and summer SST is 6 ± 2 °C and 24 ± 2 °C, respectively. The vast majority of data do not approach temperature growth limits of Mercenaria; thus, shells of the Yorktown and Chowan River formations document reduced seasonality relative to modern conditions. Population growth analyses do not provide reasonable quantitative SST estimates. They provide further evidence, however, that fossil populations likely grew in warm-temperate conditions. Isotopic results are consistent with notions of increased meridional heat transport during the Pliocene.