The 23-year-old put pen-to-paper on a two-year deal with The Black Cats, having decided in the end to join the Division One outfit following a brief trial with Premiership side Leeds United on Tuesday.
Healy agreed personal terms and passed a medical at Stadium of Light last week and, despite trying his luck with Leeds, Sunderland manager Mick McCarthy was delighted to secure the Republic of Ireland international's services.
"I'm thrilled Colin has joined us," McCarthy told the club's official website.
"There was some strong competition for his signature, including from some Premier League clubs, so it's pleasing to have secured the services of a player of his quality.
"I know Colin well and I'm fully aware of his strengths. He'll be a great addition to our squad."
The North East side are likely to pay a fee to Celtic for Healy, who cannot move on a free transfer despite being out of contract with The Bhoys - as he is under 24 years of age.
