The Brown Booby (Sula leucogaster) is a pantropical seabird that exhibits geographic variation. Brewster's Booby (S. l. brewsteri) is the most distinctive form morphologically and genetically. Until recently, Brewster's Booby was restricted to the eastern Pacific Ocean, but it is expanding westward, resulting in increasing sympatry with the Indo-Pacific form, S. l. plotus, and greater potential for interbreeding. We compiled observations of Brewster's Booby outside its usual range and we collected data on pairing patterns in the zone of overlap. At least 65 male and 53 female Brewster's Boobies have been observed on 20 islands in the central and western Pacific, with breeding documented on nine islands, mostly since 2000. Pairing by S. l. brewsteri and S. l. plotus was primarily assortative, with only a few instances of hybridization, all but one of which occurred in locations where no female S. l. brewsteri were present. The morphological differences between S. l. plotus and S. l. brewsteri appear to act as behavioral reproductive isolating mechanisms that restrict interbreeding. The morphological, genetic, and behavioral differences between S. l. brewsteri and other forms of the Brown Booby suggest it would be appropriate under all species concepts to consider Brewster's Booby as a distinct species.