Plekanec played in three junior international tournaments with the Czech Republic. He was named to the under-18 team at the 2000 IIHF World U18 Championships, where he contributed two points in six games. He then helped the Czechs to a gold medal at the 2001 World Junior Championships with two points in seven games. He made his second consecutive World Junior appearance in 2002 and improved to seven points in seven games.