Jón Ólafsson of Grunnavík (Jón Ólafsson frá Grunnavík, also known as Jón Grunnvíkingur or Grunnavíkur-Jón, 1705–1779) was an Icelandic scholar. Originally from Grunnavík, Westfjords, northwestern Iceland, he was active in Copenhagen, where he served as assistant to Árni Magnússon.
He is the author of an Icelandic dictionary and a 1732 Runologia, a treatise on runology. As in the fire of Copenhagen of 1728, the original manuscript of the Heiðarvíga saga was lost along with a recent copy made by Jón Grunnvíkingur, he wrote down a summary of the saga from memory, which is the only form in which the saga's contents survive today.
The character of Jón Grindvicensis in Halldór Laxness's historical novel Iceland's Bell is based on Jón Grunnvíkingur.