Due to climate change, tropical islands are being increasingly exposed to coastal hazards. Under energetic incident swells, infragravity (IG) waves can have a key contribution to extreme water levels and flooding, but their dynamics at barrier reef remains little studied. In this context, this study analyses IG wave generation mechanisms and transformations across a barrier reef to the southwest of Mayotte (Indian Ocean), combining a new comprehensive field dataset with phase resolving wave modelling. This analysis reveals that IG waves are mostly breakpoint-forced and suffer a substantial dissipation by bottom friction, particularly at low tide. Numerical experiments with reduced bottom friction representing a degraded coral reef suggest that IG waves would grow by about 20% across the reef at high tide. This study highlights the key role of coral reefs to dissipate IG waves and hence limit extreme water levels and subsequent coastal hazards.