Marine artificial structures (MAS), including oil and gas installations (O&G) and offshore wind farms (OWFs), have a finite operational period and require decommissioning when reaching end-of-life. Selecting the most suitable decommissioning options remains a challenge, in part because their effects are still largely undetermined. Whether decommissioned structures could act (sensu “function”) as artificial reefs (ARs) and provide desired ecological and societal benefits is of particular interest. Here, we use a meta-analysis approach of 531 effect sizes from 109 articles to assess the ecological effects of MAS, comparing O&G and OWFs to shipwrecks and ARs, with a view to inform their decommissioning. This synthesis demonstrates that whilst MAS can bring ecological benefits, important idiosyncrasies exist. In particular, we find limited conclusive evidence that O&G and OWFs would provide significant ecological benefits if decommissioned as ARs. We conclude that decommissioning options aimed at repurposing MAS into ARs may not provide the intended benefits. Hre, we provide the supplementary datasets and R code used to undertake this meta-analysis.