Seagrass meadows provide multiple ecosystem services, yet the magnitude of these services relative to unvegetated sediments and their variability across seagrass genera and regions are largely unexplored. Here, we present a global meta-analysis of three ecosystem services, organic carbon sequestration and pollution mitigation, and benthic biodiversity enhancement; synthesizing 755, 339 and 401 paired comparisons between seagrasses and unvegetated sediment, respectively. Using random-effects models and meta-regression, we assessed both the magnitude of seagrass ecosystem service enhancement and its variability across genera, tidal regimes, bioregions, and geomorphological settings. Overall, seagrass meadows increased organic carbon contents in sediments, although colonising and small seagrass species showed no significant effect on organic carbon accumulation, suggesting limited contribution to carbon offsetting efforts. Beyond serving as nutrient sinks, our analysis suggests that seagrass meadows may act as nutrient sources in oligotrophic areas, potentially playing a regulatory role in nutrient cycling. Although seagrass meadows promote microplastic and metal accumulation, the scarcity of studies with unvegetated samples as control limits the assessment of the magnitude and variability of this service. Seagrass meadows strongly enhance infaunal abundance and biodiversity, however the lack of a significant effect size on microfauna diversity highlights that both vegetated and unvegetated sediments support their own community assemblages, compositions, and functions. This meta-analysis confirms that seagrass meadows make variable contributions to these three ecosystem services, showing that seagrass ecosystem service provision is not uniform and depends strongly on biological traits and environmental contexts. Context-specific, service-specific approaches are therefore needed for ecosystem service accounting, management and restoration planning, and to avoid overgeneralisation from a small subset of well-studied seagrass taxa.