AA2024-T3 Aluminum alloy plates of 3 mm thickness were friction stir butt welded at a constant welding speed of 50 mm/min and rotation speeds of 400, 600, 800, 1000, 1250, and 1500 min⁻¹. Effects of rotation speed on microstructures, hardness distributions, and tensile properties of the joints were investigated. Equiaxed grain size increased with increasing rotation speed till 1000 min⁻¹ of rotation speed. Increase of rotation speed more than 1000 min⁻¹ brought about no significant increase of grain size in the stir zone. Also, increasing rotation speed resulted in finer and more homogenous distributions of second phase particles in the stir zone. Hardness increased both in the stir zone and thermo mechanically affected zone as the rotation speed increased and reached to that of base metal. Kissing bond-free joints were fractured at the heat affected zone on the retreating side and a maximum tensile strength of the joints was 402 MPa which was achieved at 1250 min⁻¹ of rotation speed. The joint efficiency was 88%.