The northern Appalachian Basin depocenter of Pennsylvania represents one of the most economically important hydrocarbon-producing areas in the United States, yet the thermal conditions that promoted hydrocarbon formation within the basin are only marginally constrained. The prolific coal, oil, and natural gas fields of Pennsylvania are the direct result of thermal maturation of once deeply buried organic-rich sediment. Understanding how, why, and where thermal maturation occurred in the Appalachian Basin requires high-quality heat flow and thermal conductivity measurements, as well as paleotemperature estimates and basin modeling. To improve the understanding of heat flow, we present, to our knowledge, the first direct measurements of (1) thermal conductivity on Devonian core samples and (2) equilibrium temperature versus depth logs for the northern Appalachian Basin depocenter. Results from three well sites demonstrate that heat flow is conductive and nearly uniform, averaging 34 ± 2.5 mW/m2, with an average thermal gradient of 29 ± 4°C/km. The new heat-flow measurements are significantly lower (30%–50% less) than previously published estimates that used nonequilibrium bottomhole temperature values and empirically derived thermal conductivity estimates. Our analysis indicates that previous studies correctly estimated the regional thermal gradient using bottomhole temperatures but overestimated heat flow in this region by as much as 50% because of inaccurate extrapolation of thermal conductivity. The results highlight the importance of directly measuring thermal conductivity to accurately quantify heat flow in deep sedimentary basins. Ultimately, additional paleotemperature data are necessary to improve our understanding of Appalachian Basin thermal evolution.