During his junior career, Seabrook represented Team Canada in two World Junior Championships. In 2004 in Finland, he recorded three points to help Canada to the gold medal game against the United States, where they were defeated 4–3. Returning the following year in 2005 in the United States, he matched his three-point total from the previous tournament and helped defeat Russia 6–1 in the final to capture Canada's first gold medal that began a five-year run.