In this work, friction stir welding was used to produce shear overlap joints between aluminium and a thermoplastic (AA 6111 to polyphenylene sulphide). The process uses the friction stir welding tool to create metallic chips which merge with the molten thermoplastic to form a joint. No special surface pretreatment is required before joining. Cross-sections show mechanical locking between the chipped polymer filled zone and the surrounding aluminium sheet. The effects of joining parameters such as rotational speed, translational speed and distance to backing were investigated in relation to the joint strength and failure mode. Optimum speeds and backing distances could be identified. The joint strength is dominated by mechanical interlocking between the chip and polymer filled zone and the aluminium sheet.