The transition of permafrost landscapes to marine environments, driven by climate change, plays a crucial role in the global carbon cycle. Thermokarst lagoons, formed along permafrost coasts when thermokarst lakes get connected to the sea, are key features in this transition. Using remote sensing imagery, we manually mapped and classified 520 thermokarst lagoons along the coastline of five Arctic shelf seas (Laptev, East Siberian, Chukchi, Alaskan Beaufort and Canadian Beaufort seas) between the Taymyr and Tuktoyaktuk peninsulas, and most were located along the Canadian Beaufort Sea. These lagoons cover a total area of 3457 km², with strong regional variations in both size and distribution. Based on their sea connectivity, we categorised the lagoons into five classes, with 55% in early transition stages (very low to low connected). From 2000 to 2021, lagoon area increased in all regions, with the Alaska Beaufort Sea coast showing the most growth (+1.34%). Smaller and isolated lagoons expanded faster than those in lagoon systems or deltas. Our analysis links thermokarst lagoon distribution to coastal erosion, land cover, ground ice and organic carbon, showing that most lagoons are located in areas of thermokarst lake coverage and high coastal erosion. This unique pan‐Arctic dataset serves as a foundation for understanding thermokarst lagoon dynamics and their role in the rapidly changing Arctic environment.