Coastline length is famously difficult to measure — it's a fractal, so the answer depends on the resolution of your map. The CIA World Factbook uses a standardized methodology so countries can be compared. Canada leads the world with 202,080 km of coastline, the vast majority of it Arctic.

Indonesia's 17,000 islands

Indonesia is second on the list with around 54,716 km thanks to its archipelago. Russia, the Philippines, and Japan complete the top five. Forty-five countries are landlocked and have a coastline of zero. The Vatican has no coastline; San Marino has no coastline; Bolivia famously lost its coastline to Chile in the War of the Pacific and still maintains a navy.