We present whole-rock geochemical analyses of 12 core samples obtained from three serpentinite mud volcanoes (Yinazao, Asut Tesoru, and Fantangisna) located on the forearc of the Mariana subduction system, where the Pacific Plate descends beneath the Philippine Sea Plate. The core was collected during International Ocean Discovery Program Expedition 366 of 2016-2017. The materials comprising the mud volcanoes have risen diapirically along normal faults in the forearc that may extend down to the subducting slab. Ten of the samples are thoroughly serpentinized ultramafic rocks. The other two come from cored intervals into Fantangisna mud volcano that contain materials derived from the subducting Pacific Plate, and their compositions are consistent with the inclusion of a substantial crustal component. The major, trace, and rare-earth element data reported for each analyzed core sample were obtained at the GeoAnalytical Lab of Washington State University, Pullman, Washington.