What nation controlled Fort Sumter when South Carolina seceded?

The USA - soon to be known as 'The Union' for the duration of the war - which did not recognise the newly-formed Confederate States of America.

The CSA wanted to assert its sovereignty over the island fortress, as it was in Charleston Harbour, and therefore part of South Carolina, a Confederate state. Their action in firing on Sumter is taken as the start of the war.
South Carolina declared its secession from the United States on December 20, 1860. At this time, Fort Sumter was a Union fort in the Charleston harbor.