We studied the feasibility of seawater heat extraction in Onundarfjordur (Icelandic: Önundarfjörður), a fjord located in the north-western part of Iceland, with sub-Arctic Ocean temperatures. The in-situ seawater temperature in the fjord was measured at different depths and locations from April 2015 to April 2016. Also, a 2-dimensional numerical simulation of key oceanographic parameters throughout the fjord was carried out. Measurements as well as simulations show that the seawater temperature in the fjord is generally higher than 1 °C with an upper limit of 10 °C. The simulations indicate that the area of interest is tidally dominated with higher current velocity which prominently flanks alongshore during ebb. Our findings indicate, based on oceanographic conditions of the area of study, that heat extraction from Onundarfjordur is theoretically feasible. The average seawater speed of 0.03 m/s in the fjord can gives heat power amounting to 120 kW/(m2K) when assuming natural convection through an in-situ heat exchanger. The results imply the possibility of large-scale harnessing of seawater in coastal Arctic and sub-Arctic regions, including e.g. Greenland, using heat pumps that can provide up to 80 °C hot water for space heating.