Synnejysk (Danish also Sønderjysk, High German Plattdänisch) is a dialect of the Danish language. Synnejysk is spoken in Slesvig, including Angeln. Slesvig is a part of Jutland. The other dialects classified as belonging to the Jutlandic (jysk) group of dialects are Vestjysk , Østjysk and Nordjysk .

It has been on the verge of extinction for many years, like most other Danish dialects, but a renewed popular interest to save it has been seen in recent years. This revival has been fuelled by the works of several local artists and authors, as well as Æ Synnejysk Forening, a society working for the promotion of the dialect. Several schools are now actively teaching the dialect (albeit on a smaller scale than "Rigsdansk", the official Danish).

Note that the spelling Synnejysk is in dialect, the official Danish form is Sønderjysk.