On August 31, 2015, Cousins was named the starter for the season over Robert Griffin III.
On October 4, Cousins led a 90-yard game-winning drive against the Philadelphia Eagles, his first since 2012, when he threw a touchdown pass to Pierre Garçon with under a minute left in the fourth quarter, winning the game 23–20.

On October 25, Cousins helped lead the Redskins to their largest comeback win in franchise history.
After being down 24–0 in the second quarter against the Tampa Bay Buccaneers at home, Cousins finished with 317 yards and four total touchdowns, three passing and one rushing, throwing the final touchdown pass to Jordan Reed in the final seconds of the game.
Cousins also tied a franchise record for completions in a game with 33, tying Jason Campbell, who did the same in 2007.
When heading into the locker room after the game, Cousins was caught on camera shouting "You like that?!"
to reporters, which later become a catchphrase used by Cousins, his teammates, and Redskins fans throughout the rest of the season.
Cousins and his brother Kyle applied to trademark the catchphrase later in the season, and began to sell T-shirts with the phrase to raise money for the International Justice Mission charitable organization.

Against the New Orleans Saints on November 15, Cousins threw for a career-high four touchdowns, as well as posting a perfect passer rating of 158.3, making him the first Redskins quarterback since 1950 to do that with at least 20 attempts.
Against the Buffalo Bills on December 20, Cousins passed for four touchdowns, and ran for another in a Redskins victory, marking his second four touchdown passing game of the season.

The following week against the Philadelphia Eagles, Cousins took an ill-advised quarterback kneel inside the Eagles' redzone with six seconds left before halftime, running out the clock and forgoing any chance of adding upon the Redskins' lead.
Despite that, Cousins eventually passed for four touchdowns in the game, his second consecutive four touchdown performance and third overall on the season.
The performance led to the Redskins victory over the Eagles with a score of 38–24, which allowed the team to win the NFC East division title for the first time since Griffin III did it in 2012.

Cousins finished the 2015 season posting the highest completion percentage (74.7) in home games in NFL history, with a minimum of 100 attempts, leading the Redskins to a 6–2 record at home.
He also became the first Redskins quarterback since Sonny Jurgensen in 1970 to have at least four passing touchdowns in three or more games in one season.
Against the Dallas Cowboys on January 3, 2016, he set the Redskins' single-season passing yardage record with 4,166, passing Jay Schroeder who had 4,109 in 1986.
He also finished the season with 29 passing touchdowns, second in Redskins history behind Sonny Jurgensen who had 31 in 1967, while posting a league-leading completion percentage of 69.8%, second in team history behind Sammy Baugh who completed 70.3% of his passes in 1945.

Against the Green Bay Packers in the Wild Card Round on January 10, 2016, Cousins completed 29-of-46 passes for 329 yards and two touchdowns, and was sacked six times and lost one fumble.
The Redskins lost 35–18, ending their season.
He was ranked 85th by his fellow players on the NFL Top 100 Players of 2016.
On August 31, 2015, Cousins was named the starter for the season over Robert Griffin III. On October 4, Cousins led a 90-yard game-winning drive against the Philadelphia Eagles, his first since 2012, when he threw a touchdown pass to Pierre Garçon with under a minute left in the fourth quarter, winning the game 23–20.
On October 25, Cousins helped lead the Redskins to their largest comeback win in franchise history. After being down 24–0 in the second quarter against the Tampa Bay Buccaneers at home, Cousins finished with 317 yards and four total touchdowns, three passing and one rushing, throwing the final touchdown pass to Jordan Reed in the final seconds of the game. Cousins also tied a franchise record for completions in a game with 33, tying Jason Campbell, who did the same in 2007. When heading into the locker room after the game, Cousins was caught on camera shouting "You like that?!" to reporters, which later become a catchphrase used by Cousins, his teammates, and Redskins fans throughout the rest of the season. Cousins and his brother Kyle applied to trademark the catchphrase later in the season, and began to sell T-shirts with the phrase to raise money for the International Justice Mission charitable organization.
Against the New Orleans Saints on November 15, Cousins threw for a career-high four touchdowns, as well as posting a perfect passer rating of 158.3, making him the first Redskins quarterback since 1950 to do that with at least 20 attempts. Against the Buffalo Bills on December 20, Cousins passed for four touchdowns, and ran for another in a Redskins victory, marking his second four touchdown passing game of the season.
The following week against the Philadelphia Eagles, Cousins took an ill-advised quarterback kneel inside the Eagles' redzone with six seconds left before halftime, running out the clock and forgoing any chance of adding upon the Redskins' lead. Despite that, Cousins eventually passed for four touchdowns in the game, his second consecutive four touchdown performance and third overall on the season. The performance led to the Redskins victory over the Eagles with a score of 38–24, which allowed the team to win the NFC East division title for the first time since Griffin III did it in 2012.
Cousins finished the 2015 season posting the highest completion percentage (74.7) in home games in NFL history, with a minimum of 100 attempts, leading the Redskins to a 6–2 record at home. He also became the first Redskins quarterback since Sonny Jurgensen in 1970 to have at least four passing touchdowns in three or more games in one season. Against the Dallas Cowboys on January 3, 2016, he set the Redskins' single-season passing yardage record with 4,166, passing Jay Schroeder who had 4,109 in 1986. He also finished the season with 29 passing touchdowns, second in Redskins history behind Sonny Jurgensen who had 31 in 1967, while posting a league-leading completion percentage of 69.8%, second in team history behind Sammy Baugh who completed 70.3% of his passes in 1945.
Against the Green Bay Packers in the Wild Card Round on January 10, 2016, Cousins completed 29-of-46 passes for 329 yards and two touchdowns, and was sacked six times and lost one fumble. The Redskins lost 35–18, ending their season. He was ranked 85th by his fellow players on the NFL Top 100 Players of 2016.
Kirk Cousins