<p>Hybrid aluminium-carbon fibre-reinforced polymer (Al-CFRP) composites are attracting increasing attention in high-tech aviation and automotive applications, but successfully joining them is challenging due to their differing physiochemical properties, and various surface pretreatments are applied to enhance their interfacial bonding. Herein, we develop a novel method to significantly enhance Al-CFRP bond strength by boehmite crystallisation of aluminium through thickness reinforcement (TTR) pins embedded in the CFRP matrix. The hybrid Al-CFRP joints were prepared using flat aluminium substrates and substrates with 1 mm diameter TTR pins, which were both pretreated with the boehmite crystallisation process and compared to conventional chemical etched, micro blasted, and untreated aluminium control surfaces assembled with CFRP layers in a double cantilever beam configuration for mode I testing. Results reveal that the boehmite crystallisation process can successfully grow a sea of nano needle structures on the flat aluminium surface, which significantly enhances interfacial bonding with the CFRP, leading to increases in fracture toughness of 70%, 250%, and 555% over chemical etched, micro blasted, and untreated control joints, respectively. The addition of aluminium TTR pins provide further reinforcement to the CFRP, and crystallisation of the pins increases the mode I fracture resistance of the Al-CFRP hybrid composite joints by over 1800% compared to joints made with untreated aluminium substrates.</p> Graphical Abstract <p></p>