This paper treats the twenty species and morphospecies of leaf-cutter and resin bees of the genus Megachile sensu lato known from Singapore. All five species recorded by historical workers prior to 1930 and another collected in 1961 were recorded during 2012-2014 along with 10 Megachile species newly detected in Singapore. Three of the most common species in recent surveys of urban Singapore, the resin bees Megachile (Callomegachile) umbripennis and M. (C.) disjuncta, and the leaf-cutter bee M. (Eutricharaea) sp. 1 were first detected in the country in 1974, 2010, and 1986 respectively suggesting that these they may be adventive from elsewhere in Asia rather than truly native. The following 11 Megachile are newly recorded for Singapore or were first recorded in 2013 under a different name in a study of flower-visiting insects: Megachile (Aethomegachile) borneana, M. (A.) nr. borneana, M. (A.) sp. (fusciventris group), M. (Alococanthedon) indonesica, M. (Callomegachile) ornata, M. (C.) stulta, M. (C.) sp. 1 (nr. stulta), M. (C.) sp. (biroi group), M. (Chelostomoda) moera, M. (Eutricharaea) sp. 1, M. (Paracella) tricincta. Data on occurrence within Singapore including locality, collection event, and floral host are summarised based on all available historical and recent specimens. Global distribution (including some new country and primary subdivision records), maps of Singaporean distribution, measurements, and images are provided for the named species, and new life history information about habitat associations, nesting, and floral associations are presented concisely.