DETAILED information about near-surface ocean currents is needed for effective fisheries management, pollution mitigation, search and rescue, and climate studies, but the present generation of measurement techniques provides only limited spatial and temporal resolution or coverage(1,2). In near-coastal environments, pairs of shore-based high-frequency radars have been used to map surface currents over an area of a few hundred square kilometres(3,4). The potential for mapping open-ocean current fields has been demonstrated using military high-frequency radars that can be used to ’see’ over the horizon for thousands of kilometres by reflecting signals off the ionosphere. But using one radar, only one current component can be mapped by this method(5). Here we report the mapping of surface-current vectors obtained from simultaneously employing two such radar systems with overlapping coverage. We obtain a current map in the Florida Straits, about 1,500 km from the radars, covering two 70,000 km(2) areas at a resolution of 10 km and 0.1 m s(-1). As it employs only about 2% of the radars’ potential coverage, the test shows the potential of this technique for mapping the more energetic features of ocean circulation-such as boundary currents and mesoscale eddy systems-over vast ocean areas.