Slovincian (Slovincian: slôvjinskjy, IPA: [slɵˈvjinskjɪ]; Polish: słowiński [swɔˈvij̃skʲi]; Kashubian: słowińsczi [swɔˈviɲst͡ɕi]; German: Slowinzisch) was a Lechitic language formerly spoken by the Slovincians living between lakes Gardno and Łebsko near Słupsk in Pomerania.

Slovincian is classified either as a language (first by Friedrich Lorentz), or as a Kashubian dialect or variant. Slovincian and Kashubian are both classified as Pomeranian. See below.

Slovincian became extinct in the early twentieth century ultimately due to stigmatization from Germans. However, individual words and expressions survived until after World War II, when the region became Polish. Some Slovincians were expelled along with the Germans. Of those allowed to stay, a few elderly people had fragmentary knowledge of Slovincian until the 1950s.

It is disputed whether Slovincians actually used that name, given to them by the Russian academic Aleksander Hilferding, for themselves. The synonym Lebakaschuben (Łebsko Kashubians) is also used. Some scholars believe that Slovincians regarded themselves merely as Lutheran Kashubians and their language as Kashubian. Nevertheless, the name "Slovincian" prevails in literature and is also used officially, for example in Słowiński Park Narodowy (Slovincian National Park), a protected area on the Polish Pomeranian coast.

Because Slovincian was never a written language, many different notation systems have been used in dictionaries and grammars of this language. This article will use a modified Kashubian orthography designed with northern dialects in mind used by some authors. Things such as voicing assimilation will not be accounted for to maintain an etymological spelling, as also such assimilation is predictable.

Additionally, breve ⟨˘⟩ is used for short vowels that appear in some loanwords, and macron ⟨¯⟩ is used to indicate length in one set of words. Stress is marked with ⟨ˈ⟩.

Slovincian native vowels do not show any phonemic difference in length except in one set of words; however, loanwords show unpredictability as to whether the given vowel will be long or short, giving a series of long or short loan-phonemes, existing only in loanwords.

It was originally posited that Slovincian had pitch, but it was later shown to be entirely dependent on stress, and thus, non-phonemic. However, stress itself is phonemic and can appear on any syllable. There is a tendency to place the accent on the first syllable in polysyllabic stems, especially in noun inflections and, more rarely, in verbal inflections.