Kostitsyn had a breakout season in 2007–08, playing on the Canadiens' most productive line alongside Alexei Kovalev and Tomáš Plekanec. He amassed 53 points in 78 games and was joined by his younger brother Sergei, who had been called up from Hamilton, midway through the season. Entering the playoffs with the Canadiens as the first seed, Andrei and Sergei both scored in their NHL postseason debuts against the Boston Bruins less than two minutes apart. They also both finished with eight points each in 12 playoff games. However, the Canadiens were defeated by the Philadelphia Flyers in the Eastern Conference Semifinals. On 1 July 2008, he signed a three-year extension worth an annual salary of $3.25 million with the Canadiens.