While considerable research attention has been given to co‐branding (brand alliance), empirical evidence of the success drivers remains fragmented with inconclusive findings. This meta‐analysis aims to synthesize the existing research and provide a comprehensive and generalisable set of findings. It integrates data of 197 effect sizes from 37 independent studies reported in 27 articles. The findings reveal that the relationship between the partner brands has a significantly larger impact on the success of co‐branding than the individual brand characteristics, and brand image fit is a relatively more important driver than product category fit and brand equity. Moderator analysis indicates that the relative importance of the relationship between brands is generalisable to the type of industry, business and co‐branding strategy. This paper advances theoretical understanding in three ways: (a) it increases generalisability of existing studies by investigating the impact of theoretical, contextual, and method‐related moderators on the effect sizes, (b) it brings a consensus to the equivocal findings on the importance of success drivers and (c) it identifies the knowledge gaps, and presents a future research agenda. In so doing, the paper guides practitioners by highlighting which factors to be considered and prioritised when forming a brand alliance.