A nearly complete fossil skull, including the rostrum, of blue marlin, Makaira nigricans Lacepède, 1802 (Perciformes: Xiphioidei: Istiophoridae), was collected from the Oso Member (latest Miocene) of the Capistrano Formation, Mission Viejo, Orange County, California. The specimen is compared with extant and fossil istiophorids, and 19 of its 20 morphological variables are within the range of values observed for extant M. nigricans, whereas only 13 or less variables are within the observed range of other extant istiophorids. Because extant M. nigricans usually inhabits a water column with a height of about 200 m or more and is the most tropical of all xiphioid species, its presence supports the hypotheses that the Oso Member was deposited at upper bathyal depths or greater and that the coastal paleoclimate of southern California was warmer during the late Miocene than at present.