The middle Cretaceous (125–88 Ma) greenhouse world was characterized by high atmospheric CO2 levels, the general absence of polar ice caps, and much higher global temperatures than at present. Both δ18O-based and model-based temperature reconstructions indicate extremely high sea-surface temperatures (SSTs) at high latitudes. However, there are a number of uncertainties with SST reconstructions based on δ18O isotope data of foraminifera due to diagenetic overprinting effects and tenuous assumptions with respect to the δ18O value of Cretaceous seawater, the paleoecology of middle Cretaceous marine organisms and seawater pH. Here we applied a novel SST proxy (i.e., TEX86 [tetraether index of 86 carbon atoms], based on the membrane lipids of marine crenarchaeota) derived from middle Cretaceous sedimentary rocks deposited at low latitudes. The TEX86 proxy indicates that tropical SSTs in the proto–North Atlantic were at 32–36 °C during the early Albian and late Cenomanian–early Turonian. This finding agrees with SST estimates based on δ18O paleothermometry of well-preserved foraminifera as well as global circulation model calculations. The TEX86 proxy indicates cooler SSTs (27–32 °C) for the equatorial Pacific during the early Aptian, which is in agreement with SST estimates based on δ18O paleothermometry.