Beam-to-column joints are one of the most common types of joints in metal structures. In the design of load-bearing aluminium structures, welding, as a joining method, is often avoided because of localised degradation of mechanical properties in the heat-affected zone (HAZ). However, recent experimental studies on the extent and strength of the HAZ show a significant difference compared to very conservative design rules when modern welding techniques are used. Therefore, the numerical study conducted in this paper addresses the influence of HAZ on the mechanical behaviour of the welded aluminium beam-to-column joint. Parametric numerical analyses were performed varying the aluminium alloys, the reduced mechanical properties of the HAZ, and different definitions of the HAZ extent. The obtained results show that the highest stress concentration occurs at the connection between the top beam flange and the column flange, resulting in plastic softening in this region. Different joint capacities were observed by varying the mechanical properties of the HAZ. A detailed overview of numerical models as well as the obtained moment– rotation curves show that the behaviour of some models is not as conservative as assumed in the design standards considered.