St Ledger, a Republic of Ireland international, has signed a three-month loan at the Riverside Stadium.
The Boro manager Gareth Southgate has been chasing the player all summer.
"I'm really delighted to be able to bring Sean in. He's somebody we think is already a class act," he told the club's official website.
"He reads the game really well, he's got great composure and we think he'll be a good foil for our other centre-halves."
Cardiff City's Kelvin Etuhu will miss the next two months with ankle ligaments damage.
The forward, who is on loan from the Premier League side Manchester City, suffered the injury during Sunday's 1-0 loss to Newcastle United - a match he had come off the bench in.
Watford have signed the Iceland international Heidar Helguson on loan from Queens Park Rangers.
Newcastle's owner Mike Ashley and former manager of the club Kevin Keegan will continue to attend a Premier League tribunal on Tuesday over Keegan's claim for the full payment of his contract with the club.
He quit last September, but argues his position was untenable because of former director of football Dennis Wise's handling of transfer fees.