In her freshman season (2009–10) with the Wisconsin Badgers women's ice hockey program, Decker scored the Badgers' first goal of the season in a game against North Dakota (October 3). During the season, she accumulated seven multi-point games and four multi-goal games. She was third in team scoring despite missing almost half of the first part of the season. On September 25, 2011, Decker scored her third career hat trick in a 13–0 defeat of the Lindenwood Lady Lions ice hockey program. Her 12 game winning goals during the 2010–11 season are a program record. She holds the Badgers longest scoring streak at 32 games (February 11, 2011 to January 6, 2012). During the streak she accumulated 77 points (33 goals, 44 assists).
She won gold at the 2009 IIHF World Women's U18 Championship. On January 28, 2011, it was announced that Decker was named to the preliminary roster for the U.S. Women's National Team. From April 4 to 12, 2011, she was one of 30 players that took part in a selection / training camp and was named to the final roster that participated at the 2011 IIHF Women's World Championship. At the 2011 IIHF Women's World Championship, Brianna Decker was among the tournament's top five scorers. Decker's 11 points (four goals, seven assists) placed her second.