This paper reports on a new method based on the friction stir welding process to join dissimilar metals in butt joint configuration. Two different systems were considered: AA1050 H16 aluminium/ASTM A284 steel and AA1050 H16/UNS C12200 H01 copper. The unthreaded steel tool pin was positioned in the aluminium plate so that it was tangential to the opposing metal. Bonding was accompanied by interfacial chemical reactions with no significant mechanical mixing. This new solid state welding process is called friction stir diffusion bonding. Room temperature cross-weld tensile strengths up to 82 MPa were obtained for both metal combinations. Microstructure characterisation suggested that higher joint strengths were associated with thinner, <1 μm thick intermetallic reaction layers at joint interfaces.