Kostopoulos was drafted by the Pittsburgh Penguins in the seventh round of the 1999 NHL Entry Draft. Before that he played junior hockey with the London Knights of the Ontario Hockey League. He made his professional minor-league hockey debut with the Wilkes-Barre/Scranton Penguins of the American Hockey League. Kostopoulos made his NHL debut with the Penguins in the 2001–02 season and enjoyed his best season so far with Pittsburgh in 2003–04. During the 2005 off season, he played with the Manchester Monarchs, having signed a free-agent contract with the Los Angeles Kings. On July 3, 2007 he signed a free-agent contract with the Montreal Canadiens.