McCown started the Week 1 game against the New York Jets. At the end of his first drive, he made a 13-yard run and tried to jump into the endzone, but he fumbled inches away from the goal line and was taken out of the game due to a concussion. McCown returned from his injury in a Week 3 game against the Oakland Raiders on September 27, where he threw for 341 yards, 2 touchdowns, and 1 interception as the Browns lost by a score of 27–20. His brother Luke also started for the New Orleans Saints the same day, marking the first time both McCown brothers had started on the same day since December 9, 2007. Josh then threw for 356 yards and 2 touchdowns against the San Diego Chargers but the Browns lost by a score of 30–27. In a 33–30 overtime victory over the Baltimore Ravens in Week 5, he set a new career high and the Browns record for passing yards in a regular season game with 457 as the Browns moved to 2–3 while McCown was named the AFC Offensive Player of the Week. This was the Browns' first victory in Baltimore since the 2007 season. McCown also became the first player in NFL history to record more than 450 passing yards, two passing touchdowns, a rushing touchdown, and no interceptions in one game. He was also the first player in Browns history to pass for more than 300 yards in three straight games and his 1,154 yards set a record for most in a three-game span in franchise history, surpassing the former record of 1,038 set by Brian Sipe in 1980. The jerseys that McCown and teammate Gary Barnidge wore during the Week 5 Ravens game were sent to, and displayed, at the Pro Football Hall of Fame. McCown then started the next three games of the season. During the second game of that stretch against the St. Louis Rams, McCown hurt his ribs on a hard hit but stayed in the game for three more plays before suffering a shoulder injury and leaving the game with 5:17 left in the fourth quarter. On October 30, prior to the game against the Arizona Cardinals on November 1, McCown was listed as questionable with shoulder and rib injuries. He ended up starting the game, passing for 211 yards and a season-high 3 touchdowns, which all came in the first half, as the Browns lost by a score of 34–20. McCown also took several hard hits early in the third quarter, exacerbating his rib injury from the previous week. However, he stayed in the game until leaving late in the fourth quarter. Due to McCown's injury, Johnny Manziel started the next two games. On November 17, Manziel, coming off a 372-yard performance against the Pittsburgh Steelers, was named the starter for the rest of the season, but McCown was promoted to starter again after a video surfaced online of Manziel partying. In the ensuing Week 12 rematch against the Ravens on December 1, McCown broke his collarbone late in the third quarter and stayed in for two plays before leaving the game. On December 2, it was announced that McCown would miss the rest of the season with the injury. He was placed on injured reserve the same day. He threw for 2,109 yards, twelve touchdowns, and four interceptions while also rushing for 98 yards and one touchdown in eight games during the 2015 season. The Browns finished the year with a 3–13 record and were 1–7 in games that McCown started, despite him posting a passer rating of 93.3 on the season. He was the Browns nominee for the Art Rooney Sportsmanship Award in 2015. The local chapter of the Pro Football Writers of America named McCown its Dino Lucarelli Good Guy Award winner.