A premixer for a gas turbine combustor includes a centerbody, a swirler assembly, and a mixing duct. The swirler assembly includes an inner swirler with vanes that rotate air in a first direction and an outer swirler with vanes that rotate air in an opposite direction. The inner swirler vanes and the outer swirler vanes are separated by an annular splitter. The outer swirler vanes define an outlet plane, and the inner swirler vanes each have a trailing edge that is disposed at an acute angle relative to the outlet plane. In one aspect, the inner swirler is axially offset from the outer swirler. The mixing duct may also define fuel passages that deliver fuel to fuel outlets on the downstream end of the mixing duct. The premixer is designed for operation on gaseous fuel or liquid fuel.