Ensign during the series, Lieutenant during The Motion Picture, Commander from The Wrath of Khan - Generations, and ensign again in the new movie. Also I believe that he should become a captain after Generations, but that has not happened yet. (Perry Hammett's answer)True,In 2286, Chekov took command of the Enterprise-A and posed as "Captain Chekov" as a ruse to negotiate with Sybok for the hostages that the renegade Vulcan took on Nimbus III, while Kirk, Spock, Uhura, and a security team landed on the planet covertly by shuttlecraft. Chekov successfully distracted Sybok long enough for the landing party to launch an attack on Paradise City.(Star Trek V: The Final Frontier)
In the William Shatner novels, Chekov was referred to as an admiral, former captain of th starships Potemkin and Cydonia. The "Vulcan's Soul" books expand that he also commanded USS Undaunted, and was at one time the commander-in-chief of Starfleet and retired at the rank of fleet admiral.