Sergei Kostitsyn (Belarusian: Сяргей Касьціцын, Siarhiej Kascicyn; born March 20, 1987 in Navapolatsk, Byelorussian SSR, Soviet Union) is a Belarusian professional ice hockey winger currently playing for the Montreal Canadiens of the National Hockey League (NHL) along with his older brother, Andrei.

Like his older brother, Sergei excelled in the Belarusian junior hockey program. Sergei was one of the leading scorers for HK Gomel in Belarus and was drafted in the Canadian Hockey League (CHL) Import Draft by the Ontario Hockey League (OHL)'s London Knights. Sergei was also drafted in the seventh round, 200th overall in the 2005 NHL Entry Draft by the Montreal Canadiens, the team that also had drafted Andrei in the first round (10th overall) in the 2003 NHL Entry Draft.

Kostitsyn moved to North America in 2005–06 to play with the Knights and recorded 26 goals, 52 assists and 78 points, leading all OHL rookies in scoring during the 2005–06 season. The 19-year-old was the second runner-up for the Emms Family Award, as OHL Rookie of the Year, behind Logan Couture of the Ottawa 67's and 15-year-old winner John Tavares of the Oshawa Generals. Sergei was instrumental in the London Knights' 2006 playoff run, which saw them lose to the Peterborough Petes in the OHL final. In his first ever OHL playoff game against the Sault Ste. Marie Greyhounds, Sergei had 6 points, a goal and 5 assists, in a 6–1 London win. In the semi-finals, Kostitsyn scored the series-clinching goal against the Guelph Storm, a power-play goal in the first overtime of game five. In addition to being named team rookie of the year, the Knights named him the playoff performer of the year, finishing the post-season with 37 points in 19 games.