The Tunisia international has signed a deal until the end of the season with the Ligue 1 title challengers, who have the option to sign the 24-year-old on a four-year deal following the end of his loan spell.
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, AS Monaco FC is a great club, very professional, with excellent staff, great players and great ambition," he told the club's official website.
"I am ready to help my team-mates. I will do everything to succeed. We have a special purpose."
'Obviously Messi is the best player in history and being compared to him means that you're doing things right, but I try to be myself': Barcelona star responds to comparisons with Argentine legend
'Darwin Nunez would be gold for Arsenal': Liverpool legend claims Gunners should have signed Uruguayan striker in January
