2008-09: Petter Granberg played for Skelleftea at the J18 level. In 32 games for that team, Granberg picked up 8 assists and 20 penalty minutes. He had no points in 8 playoff games.
2009-10: Played for Skelleftea in the J20 SuperElit league, posting 2 goals and 7 assists for 9 points in 40 games. He picked up 1 goal in 4 playoff games for that squad. In the U-18 World Junior Championships, he had 1 assist in six games for Sweden. Granberg is one of the youngest prospects in the 2010 draft. Granberg was selected 116th by the Maple Leafs in 2010. He was ranked 144th by ISS and 21st by Central Scouting for European skaters. He was the second highest rated Swedish defender by Central Scouting.
2010-11: Granberg saw limited playing time in 22 games for Skelleftea in Sweden's Elitserien; spending most of the season with the club's U20 squad. Granberg averaged just over five minutes of ice time per game and had 1 assist with 6 PMs for third-place Skelleftea. Granberg had 1 assist with 2 PMs skating in 11 of 18 playoff games as Skelleftea reached the Elitserien finals. In 34 U20 games he scored 2 goals with 6 assists and had 16 PMs. Granberg skated in seven international matches for Sweden's U19 and U20 national teams but did not compete in the 2011 U20 World Junior Championship.
2011-12: Granberg's role steadily grew for in his first full season in Sweden's Elitserien and he was part of Sweden's gold medal-winning team at the 2012 U20 World Junior Championship. He appeared in 38 of 55 regular season games for Skelleftea; finishing minus-one while averaging just over 13 minutes of ice time, and scored 1 goal with 3 assists and 10 penalty minutes. Skelleftea was second in the regular season and advanced to the playoff finals. Granberg averaged 15:36 minutes of ice time in 19 playoff games and was minus-two with 1 goal and 1 assist; accumulating 12 penalty minutes. In five games for Sweden at the WJC he had an even plus/minus with 1 assist and 4 penalty minutes. Granberg signed a three-year, entry-level contract with the Maple Leafs in May, 2012.
2012-13: Granberg finished the year with a flourish after missing much of the season due to injury — skating for Elitserien champion Skelleftea in the playoffs and then winning a gold medal with Sweden's national team as a 20-year-old. He played in 13 games for Skelleftea during the regular season and was +1 with no points and 4 penalty minutes; averaging 13 minutes of ice time. After finishing first in the regular season, Skelleftea lost just once in 13 playoff games. Granberg skated in all 13 games and was +7 with 2 assists and 10 penalty minutes; averaging 16 minutes of ice time. In 10 games at the 2013 IIHF World Championship he had an even plus/minus with 1 assist and 2 penalty minutes. Sweden defeated Switzerland, 5-1, in the gold medal game.
A defensive defender with very good size, Granberg has both strong defensive acumen and can chip in with some offense from time to time with a strong first-pass. He plays a very smart defensive game and is rarely caught out of position. Does a very good job containing opposition forwards in the defensive zone due to his large wing span and body position. Compared in style to current Maple Leaf defender Carl Gunnarsson, Granberg is said to have a bit more of a physical edge to his game. He is expected to become an even more dominant defender as he matures.
Granberg will compete for a roster spot with the Maple Leafs, but is likely to spend a large chunk of the season with the Toronto Marlies in 2013-14.