Which State Is Closest to the Atlantic: Massachusetts, Mississippi, or Missouri?

Massachusetts—by a wide margin.

Why

• Massachusetts: Entire eastern border is Atlantic coastline (Boston, Cape Cod, Islands). Distance to ocean = 0 miles.
• Mississippi: Gulf Coast state (Atlantic via Gulf of Mexico). ~100 km (60 mi) of Gulf beaches. Distance to open Atlantic = ~500 km (via Florida Straits).
• Missouri: Landlocked, central U.S. Closest Atlantic access: ~1,300 km to Virginia/North Carolina coast.

Key Distinction

"Atlantic Ocean" geographically includes marginal seas (Gulf of Mexico, Caribbean, Mediterranean). But colloquially, "Atlantic coast" means the eastern seaboard. Massachusetts is on it; Mississippi is on a gulf; Missouri is inland.