The Aleocharinae (Coleoptera: Staphylinidae) of Ontario were reviewed in the context of recently studied material, primarily from insect surveys conducted by the University of Guelph Insect Collection (Ontario, Canada). Aleochara daviesi Klimaszewski & Brunke sp. n., Agaricomorpha websteri Klimaszewski & Brunke sp. n., Atheta (Microdota) alesi Klimaszewski & Brunke sp. n., Dinaraea backusensis Klimaszewski & Brunke sp. n., and Strigota obscurata Klimaszewski & Brunke sp. n. are described as new to science. We also report 47 new Ontario records and 24 new Canadian records. Callicerus rigidicornis (Erichson) and Alevonota gracilenta (Erichson) are newly reported from North America as adventive species. A checklist, with Canadian distributions by province, of the 224 species of Aleocharinae known from Ontario is given. The following species are placed in subjective synonymy with Dexiogyia angustiventris (Casey): (Dexiogyia asperata (Casey) syn. n., Dexiogyia abscissa (Casey) syn. n., Dexiogyia tenuicauda (Casey) syn. n., Dexiogyia intenta (Casey) syn. n., Dexiogyia alticola (Casey) syn. n.). The following species are placed in subjective synonymy with Acrotona subpygmaea (Bernhauer): (Acrotona avia (Casey) syn. n., Acrotona puritana (Casey) syn. n.). Lectotypes are designated for Thiasophila angustiventris Casey, Thiasophila asperata Casey, Ischnoglossa intenta Casey, Oxypoda rubescans Casey, Chilopora americana Casey, Chilopora fuliginosa Casey, Coprothassa smithi Casey, Atheta subpygmaea Bernhauer, Colpodota puritana Casey, Strigota seducens Casey, Trichiusa compacta Casey, Trichiusa hirsuta Casey and Trichiusa robustula Casey.
Keywords: Aleocharinae; Canada; Ontario; biodiversity; distributional records; taxonomy.