The sprawl of marine construction is one of the most extreme human modifications to global seascapes. Nevertheless, its global extent remains largely unquantified compared to that on land. We synthesized disparate information from a diversity of sources to provide a global assessment of the extent of existing and projected marine construction and its effects on the seascape. Here we estimated that the physical footprint of built structures was at least 32,000 km² worldwide as of 2018, and is expected to cover 39,400 km² by 2028. The area of seascape modified around structures was 1.0–3.4 × 10⁶ km² in 2018 and was projected to increase by 50–70% for power and aquaculture infrastructure, cables and tunnels by 2028. In 2018, marine construction affected 1.5% (0.7–2.4%) of global Exclusive Economic Zones, comparable to the global extent of urban land estimated at 0.02–1.7%. This study provides a critical baseline for tracking future marine human development.