Grand Lake St. Marys Seaplane Base (FAA LID: O12) is a public use seaplane base located five nautical miles (9 km) southwest of the central business district of St. Marys, a city in Auglaize County, Ohio, United States.

The airport is a significant meeting place for the Ohio Seaplane Pilots Association.

[2] Grand Lake St. Marys Seaplane Base covers an area of 643 acres (260 ha) at an elevation of 871 feet (265 m) above mean sea level.

It has one seaplane landing area designated 9/27 and measuring 7,000 x 4,000 ft (2,134 x 1,219 m).

For the 12-month period ending September 27, 2022, the airport has 12 aircraft operations per year, all general aviation.