The 26-year-old spent five years at Dortmund, but made just 22 Bundesliga appearances as understudy to Borussia's German squad member Roman Weidenfeller.
Langerak finished with premiership medals when Borussia won the 2011 and 2012 Bundesliga titles and German Cup in 2012, when he came off the bench to play his part in a 5-2 win over Bayern Munich, but wants more regular football.
Having won six caps for Australia, Langerak completed his transfer to Stuttgart on Monday and jumped straight into training at his new club's pre-season camp in Austria.
"Stuttgart was always a tough opponent, who always fought hard against Dortmund. The team has quality and I am proud to be part of this team," Langerak told the Stuttgarter Nachrichten.
"I am aware that I need to work hard in pre-season so that I can play.
"When things get going again, we want to start our season well - then we'll see what's possible."
Stuttgart faces Cologne in its opening match in August when the new season begins.
He will be fighting with Poland international Przemyslaw Tyton and Germany Under-20 goalkeeper Odisseas Vlachodimos for the No.1 shirt at Stuttgart, who only narrowly avoided relegation last season to finish 14th on the table, while Dortmund was seventh.
"For goalkeepers, it is quite normal that they have to compete with one or two good rivals and we all have to fight for a regular place," said Langerak.
"I expected this situation and I am looking forward to the challenge."
