Untxillaitz, Untxillaitx, or Untzillatx, is a mountain of Biscay, Basque Country (Spain), 934 m. high.

It forms the pass of Atxarte with the neighbouring Aitz Txiki and it is located in the valleys of Mendiola and Mañaria.

(1h 30m) From the neighbourhood of Zelaieta in Abadiano the road goes to the Atxarte pass there the river is crossed and a path turns to the right and there is a steep ascension beside the Urresti prominence.

At the top of Urresti there is a difficult rock climb that leads to the Elosua col route.

(2h) From the quarry at this side of the mountain, there is a path that leads to the col of Elosua, from where the summit is reached following the crest.