Sisko was promoted to Captain on stardate 48959 during the episode "The Adversary".

Was Benjamin Sisko a captain?

He had “captained” The Defiant several times while still holding the rank of commander, which made him captain of the ship when on board (nobody addressed him that way, although they technically could have), but he wasn't Captain Sisko until being decorated as such by Starfleet.

Who is the captain in Deep Space Nine?

Capt. Benjamin Sisko ("Star Trek: Deep Space Nine," 1993-1999)