Hydrotreated renewable diesel (HRD) is a new type of biodiesel which has been hydroprocessed to remove oxygen, resulting in a composition comparable to that of petroleum diesel. A 50%-50% blend of algal HRD-76 and petroleum diesel F-76 has been successfully used in warships as a demonstration by the U.S. Navy. The present work studies corrosion of plain-carbon steel (UNS G10180(1)) in natural seawater/fuel mixtures of F-76, HRD-76, and a 50%-50% blend of F-76 and HRD-76 (50-50 blend). Severe corrosion occurred on steel immersed in the seawater layer with the outer rust layer identified as lepidocrocite and the inner rust layer identified as magnetite. The magnetite layer that formed in the seawater/HRD-76 mixture appeared thicker than those formed in the other seawater/fuel mixtures, indicating more severe corrosion. Red rust formed on the steel samples immersed in the fuel layer for F-76 and 50-50 blend cases, while white deposits of carbonate type minerals formed in the HRD-76 layer. The red rust formation in the fuel layer was possibly due to stronger acidity of F-76 than HRD-76, while the formation of white deposits suggests strong cathodic activity on steel immersed in HRD-76 and therefore severe corrosion on steel in the corresponding seawater phase.