The 27-year-old defender has been on trial at the Sky Bet Championship club for the last five weeks and will now join compatriot Gustav Engvall - a deadline day capture from IFK Gothenburg - in Lee Johnson's squad.
Ekstrand, who like Engvall has won two caps for his country, was available on a free transfer after his contract with Watford expired in the summer.
City boss Johnson told bcfc.co.uk: "Joel is a top player, full of Sky Bet Championship experience.
"We have had him in training with us and have assessed his old injury. Myself and my staff are confident he will be an excellent signing for the club."