We make use of the Community Atmosphere Model version 5 Green's function q‐flux perturbation experiments to explore the most effective remote forcing in driving U.S.‐wide summer heat extremes. We find that positive q‐flux forcing over the western North Pacific Ocean is the most effective in causing an increased heat extreme frequency. This works by driving increased sea surface temperature and precipitation over western North Pacific and an eastward propagating Rossby wave train with an anomalous ridge over the contiguous U.S. In comparison, negative q‐flux forcing over the eastern tropical Pacific and its resulting surface cooling also leads to an increased heat extreme frequency but is less effective. Furthermore, guided by the Green's function results, we separate the role of western North Pacific warming and eastern tropical Pacific cooling in U.S. heat extremes in prescribed sea surface temperature experiments and ERA5 reanalysis data and find overall consistent conclusions. Plain Language Summary We study summertime U.S. heat extremes and their precursors in ocean surface forcing. Using a novel approach, we find that western North Pacific warming and wetting is the most effective way to cause an increased frequency of U.S.‐wide heat extremes. In comparison, eastern tropical Pacific cooling can also lead to an increased heat extreme frequency but is less effective. Our work advances understanding of U.S. heat extremes with important implications for their predictability. Key Points Green's function experiments separate the role of western North Pacific warming and tropical Pacific cooling in causing U.S. heat extremes Green's function experiments highlight the dominant role of western North Pacific warming in causing U.S.‐wide heat extremes Composite analysis based on prescribed sea surface temperature experiments and ERA5 supports the conclusion