The Tunisia international has signed a deal until the end of the season with the Ligue 1 title challengers, who will then have the option to sign the 24-year-old on a four-year contract.
Abdennour has made 82 appearances for Toulouse, earning a reputation as one of the more talented defenders in the French game, and the centre-back, who can also operate on the left side of the defence, was quick to express his delight at securing a move to the Stade Louis II.
"I am very proud, Monaco are a great club, very professional, with excellent staff, great players and great ambition," he told their official website.
"I am ready to help my team-mates. I will do everything to succeed. We have a special purpose."
Abdennour follows in the deadline-day footsteps of Dimitar Berbatov, who has joined on loan from Fulham for the remainder of the 2013-14 campaign.