Cook captained England in the 2015 Ashes series, and although England won the first game, Cook could only manage scores of 20 in the first innings and 12 in the second. England lost the second Test by 405 runs, but Cook was one of the few England batsman to show any resistance, scoring 96 in England's first innings. With the series level Cook made 34 in England's first innings of the third Test and England went on to win the game by eight wickets. England won the fourth Test by an innings and 78 runs, meaning Cook only needed to bat once, scoring 43 in England's first innings. With the Ashes won, Cook was praised for his captaincy and he gave an emotional interview following the victory. England lost the final match of the series, but Cook was again one of few England batsmen to show any resistance with the bat as he scored 85 in their second innings. Despite the loss, England won the series 3–2 and Cook celebrated his second Ashes win as captain.
Cook scored his third double-century and 28th Test ton (263) against Pakistan in first Test at Abu Dhabi in October 2015 in 836 minutes: the longest innings by an Englishman and 3rd longest of all time, facing 528 balls. In doing so he also became the all-time highest non-Asian run scorer in Asia, overtaking Jacques Kallis' 2,039 runs. Despite Cook's brilliant innings, England were unable to force a victory, running out of time after bad light stopped play. In the second Test Cook scored 65 in the first innings as England posted 242. With England chasing down a big target to win in the second innings, Cook could only manage 10 and Pakistan won the match by 127 runs. England need to win the third Test to have any chance of drawing the series and despite taking a first innings lead which Cook contributed 49 to, Pakistan batted well in the second innings and England were dismissed for 256 in their second innings, meaning the lost the series 2–0, despite Cook scoring 63.
For his performances in 2015, he was named as captain of the World Test XI by the ICC. He was also named in the Test XI by ESPNCricinfo and Cricbuzz.