Michael Hector has committed his future to Reading after signing a new three-year contract with the Championship club.
Watford have confirmed the signing of Austria defender Sebastian Prodl from Werder Bremen.
The 18-year-old Norway youth international has put pen to paper on a deal until June 2016 and the Terriers have the option of an extra year.
Town manager Mark Robins told the club's official website: "As part of our scouting activity in Scandinavia we watched Sondre on a number of occasions for IK Start as well as on international duty with Norway before we invited him to spend a week with us in late November 2013 after his season and contract ended.
"He impressed me and our staff during that training period. He has great technical ability and a superb passing range.
"He's a central midfielder and is able to play as a deep-lying player or as a more advanced 'number 10'.
"Sondre is only 18 and we believe can develop further with us. He has a real desire to learn and do well, so we're excited about working with him over the coming weeks and months.
"We are building a group of young players, through a balance of careful acquisition and developing our academy, which is getting stronger, with lots of potential and incorporating types that fit the long-term philosophy of the club."
Tronstad made his debut for IK Start at 16 and has made a total of 23 appearances, including nine since they won promotion to the Norwegian top flight last season.