The Onyar (Catalan: [uˈɲa]; Spanish: Oñar [oˈɲaɾ] ; Undarius in Latin) is a Spanish river in Girona, Catalonia that begins at the Guilleries massif at the apex of the Catalan Transversal Range and the Pre-Coastal Range. It joins the Ter at the city of Girona, and crosses the city from south to north, separating the city into the old part (on the right side) and the more modern part (on the left side).
During its course through the city, the Onyar river is joined by the waters coming from the Monar or Conda irrigation channel (which in turn comes from the Ter river), a channel that used to irrigate the orchards in the "Mercadal" area. As it journeys further north, the river converges with the Güell and the Galligans rivers near the crossing with the railway. Finally, it merges into the Ter river near the Pedret neighborhood.