Saterland Frisian, also known as Sater Frisian, Saterfrisian or Saterlandic (Seeltersk [ˈseːltɐsk]), spoken in the Saterland municipality of Lower Saxony in Germany, is the last living dialect of the East Frisian language. It is closely related to the other Frisian languages: North Frisian, spoken in Germany as well, and West Frisian, spoken in the Dutch province of Friesland.

From a diachronical perspective, Saterland Frisian is an Emsfrisian dialect of the East Frisian language. Emsfrisian used to be spoken in the western half of the East Frisian peninsula and in the Ommelanden. The other East Frisian dialect group was the Weserfrisian, formerly spoken from the eastern half of the East Frisian peninsula to beyond the Weser.

Together with West Frisian and North Frisian it belongs to the Frisian branch of the Germanic languages. The three Frisian languages evolved from Old Frisian. Among the living Frisian dialects, the one spoken in Heligoland (called Halunder) is the closest to Saterland Frisian. The closest language other than Frisian dialects is English.

Frisian and English are often grouped together as Anglo-Frisian languages. Today, English, Frisian and Lower German, sometimes also Dutch, are grouped together under the label North Sea Germanic. Low German, which is closely related to Saterland Frisian, lacks many North Sea Germanic features already from the Old Saxon period onward. In turn, Saterland Frisian has had prolonged close contact with Low German.

Settlers from East Frisia, who left their homelands around 1100 AD due to natural disasters, established the Frisian language in the Saterland. Since the sparse population at the time of their arrival spoke Old Saxon, the Frisian language of the settlers came into close contact with Low German.

In East Frisia, the assimilation of Frisian speakers into the Low German speaking population was well under way in the early 16th century. The dialect of the Saterland persisted mostly due to geography. As the Saterland is surrounded by bogland, its inhabitants had few contacts with adjacent regions. The villages built on sandy hills were basically like islands. Until the 19th century, the settlement area was almost exclusively reachable by boat via the river Sagter Ems (Seelter Äi), the exception being walking on frozen or dried out bogland during times of extreme weather.

Politically, the land did not belong to the County of East Frisia, which came into existence in the 15th century, but changed hands frequently until it became part of the County of Oldenburg. The resulting border was not merely political, but also denominational, as the Saterland was recatholicized. The Saterland was also linguistically and culturally different from Oldenburg. This led to further isolation.

Colonialization of the bogland, with construction of roads and railways, led to the Saterland being less isolated. Nevertheless, Saterfrisian survived, because most of the community living in the Saterland continued to use the language. This common linguistic area was disturbed following World War II. German repatriates from Eastern Europe were settled in the Saterland, leading to Standard German gradually replacing Saterfrisian. While the predicted language death in the late 20th century did not happen, and the number of speakers remained stable, the Saterfrisian speaking community nowadays make up only a minority of the Saterland population.