To improve the strength of the adhesive-bonded aluminum alloy, atmospheric pressure plasma treatment (APPT) was conducted on aluminum AA5052 with three different gases (i.e., argon, nitrogen and air). The contact angle and surface free energy of the as-received and treated AA5052 were measured and mechanical testing of the adhesive-bonded joints were performed. Test results indicated that air and nitrogen plasma treatments significantly improved the wettability of AA5052, and consequently resulted in the increase of joint strength. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used for the characterisation of the aluminum surface physical and chemical changes induced by plasma treatment. It was found that APPT with gases used in this study hardly induced noticeable difference on the surface morphology of aluminum AA5052. The results of XPS indicated that argon plasma treatment removed the residual carbon contamination without significant affecting the chemical state of oxide layer. A thick aluminum oxide layer with adsorption of polar groups (i.e., -NH2, -NO3, -OH) was formed on the nitrogen and air plasma-treated surfaces of aluminum AA5052, which was likely a major contributing factor to the improved wettability and bond strength in adhesive bonding of aluminum AA5052.