Neil Lennon says it is "an honour" to have been asked to manage Celtic for a second time after Brendan Rodgers completed his move to Leicester City.
Lennon, 47, will take charge until the end of the season, starting with Wednesday's Scottish Premiership game against Hearts at Tynecastle.
The Northern Irishman returns to the club he led to three titles from 2011, having left Hibernian last month.
"This is a club which has been a huge part of my life already," Lennon said.
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Lennon, who will be assisted by current first-team coach John Kennedy and former Republic of Ireland international Damien Duff, also won two Scottish Cups during four years at Celtic Park.
Prior to that, the former midfielder made 214 appearances for the club in a seven-year spell before leaving in 2007.
"There's a chasm to fill with Brendan leaving," Lennon told Celtic TV.
"He's done a remarkable job, but I'm here for the club. The club comes first as do the players. I want to bring as much success during my time here as I possibly can.
"All I want to do is carry that on - it's not broken and I'm not coming in and looking to revolutionise anything."
'He knows what it means to be Celtic manager'
Rodgers has signed a contract with Leicester until June 2022 and succeeds Claude Puel, who was sacked on Sunday after 16 months in charge.
It marks a Premier League return for Rodgers, who previously managed Liverpool and Swansea City.
Lennon, meanwhile, left Hibs in controversial circumstances in January, with the Scottish Premiership side saying he had "not been dismissed" and had "not resigned".
He left Celtic in 2014, before an ill-fated stint with Bolton Wanderers in the English Championship.
"In bringing Neil back until the end of the season, we have appointed a man who knows what it means to be Celtic manager and someone who knows how to win," Celtic chief executive Peter Lawwell said.
"We know he has the commitment and desire needed to face the challenges ahead.
"We are delighted to also promote John Kennedy to assistant manager and bring Damien Duff into our first-team management group. This will bring vital continuity and allow us to build on Brendan's legacy."
'Appointment steadies the ship' - analysis
Former Celtic striker Scott McDonald
It's very good from a Celtic point of view that they've been able to bring in someone so quickly to steady the ship. As a player, you always want to know and see your leader and to have someone come in straight away. There are still players there who had Neil as a manager and I'm sure they'll be sending the message out to the other players as to what type of manager he is.
What he knows about that football club is everything, so I think at this moment in time it's the perfect appointment. I can't imagine him coming in and changing too much. I think he'll just go with what has been successful and the man-management skills he's got are key in the run-in to the end of the season.