The Old World genus Xenomerus Walker is revised. Thirty one (31) species are recognized based on 879 specimens. Twenty four (24) new species are described: X. armatus (Oriental), X. aureipes (Ethiopian), X. bickeli (Australian), X. comatus (Ethiopian), X. fulleri (Australian), X. gloriosus (Australian), X. guinensis (Australian), X. halteratus (Australian), X. hilleri (Australian), X. feehani (Ethiopian), X. kalocsai (Ethiopian), X. malawi (Ethiopian), X. melikai (Australian), X. melleus (Australian), X. noyesi (Oriental), X. ochraceus (Ethiopian, Oriental), X. orientalis (Oriental), X. parorientalis (Oriental), X. rugifrons (Oriental), X. scutellatus (Ethiopian), X. spinosus (Oriental), X. vanharteni (Ethiopian), X. watshami (Ethiopian) and X. yamagishii (Oriental, Palaearctic). Redescriptions and new combinations for the following species are provided: Xenomerus buccatus (Kononova & Kozlov) from Trimorus; Xenomerus calligetus (Kononova & Kozlov) from Trimorus; X. cornutus (Kononova & Kozlov) from Trimorus. Xenomerus canariensis Huggert, X. ergenna Walker, X. laticeps Dodd and X. varipes Dodd are redescribed. New synonymies are proposed: Trimorus mutator Kononova & Kozlov = X. canariensis Huggert, Trimorus curtum Kononova & Kozlov = X. ergenna Walker, Xenomerus hibernicus Mineo & O’Connor = X. canariensis Huggert. Xenomerus latimetascutum Szabo is transferred to Trimorus. Xenomerus atomus Rajmohana & Narendran, Xenomerus indicus Mukerjee, Xenomerus solox Kozlov et Lé, Xenomerus forax Kozlov et Lé and Xenomerus flavicornis Dodd are considered species of uncertain status (holotypes not aviable). An identification key is provided, and four species groups are proposed.