A taxonomic revision of the genus Gouania Jacq. (Rhamnaceae) is presented for Madagascar and the other western Indian Ocean islands. Seventeen species are recognized, of which nine are described and published as new (all endemic to Madagascar): G. ambrensis Buerki, Phillipson & Callm., G. callmanderi Buerki, G. cupreifolia Buerki, Phillipson & Callm., G. cupuliflora Buerki, Phillipson & Callm., G. gautieri Buerki, Phillipson & Callm., G. perrieri Buerki, Phillipson & Callm., G. phillipsonii Buerki, G. taolagnarensis Buerki, Phillipson & Callm., and G. zebrifolia Buerki, Phillipson & Callm. Sixteen species occur in Madagascar, of which 13 are endemic and three are common to Madagascar and one or more of the smaller Indian Ocean islands. The latter include G. laxiflora Tul., a species which is also present on mainland Africa. One species, G. mauritiana Lam., is endemic to Réunion Island. We recognize two subspecies within G. scandens (Gaertn.) R. B. Drumm.: G. scandens subsp. scandens and G. scandens subsp. glandulosa (Boivin ex Tul.) Buerki, Phillipson & Callm., the latter transferred from G. glandulosa Boivin ex Tul. Past confusion about the identity of this species is discussed. Five names are lectotypified: G. aphrodes Tul., G. glandulosa [= G. scandens subsp. glandulosa], G. laxiflora, G. lineata Tul., and G. tiliifolia Lam. Both lectotype and epitype are designated for G. mauritiana. Conservation assessments are provided for all species within their primary areas of occurrence.