Tosa dialect (土佐弁, Tosa-ben) is spoken in the central and eastern regions of modern-day Kōchi Prefecture in Japan. This area was once known as the Tosa area and the name of the dialect still reflects this older name. The dialect itself developed from the linguistic influence of immigrants to the Tosa area from other parts of Japan and is largely a combination of the Kansai dialect and Chugoku dialects. In particular the accent system used is that of old Kansai-ben.
One of the distinctions between Tosa dialect and other Western Japanese dialects is the use of the verb suffixes -yuu and -chuu to show aspect. Grammatically, -yuu encapsulates the continuative and progressive aspects while -chuu separately encapsulates a repeated action or continuing state. In Standard Japanese both are represented by -te-iru. The respective past tense forms are -yotta and -chotta, while the negative forms are -yasen and -chasen.
Other distinctive features of Tosa dialect include its clause-ending particles, such as kendo for disjunction, ga as a question marker and ki to denote reason.
Many features of Tosa dialect are shared with neighbouring Hata dialect, spoken in the western part of Kōchi Prefecture.