The Millers moved quickly following the decision to sack Alan Stubbs after Tuesday night’s defeat at Birmingham City, the club’s sixth in a row.
Jackett has signed a three-year contract and will take charge of tomorrow’s game at home to Reading.
The 54-year-old is United’s fourth appointment since Steve Evans was sacked a little over a year ago.
He arrives at the New York Stadium with a wealth of managerial experience following spells with Watford, Swansea City, Millwall and Wolverhampton Wanderers.
Chairman Tony Stewart said: “It was probably the best interview I have had with somebody since I took over the club.
“Kenny was available and we acted swiftly because we knew that he has got the experience, he has managed a great club like Wolves and if you check the stats out his record is superb.
“We had a good feeling he would be right, then when we sat down to interview him I liked his enthusiasm. We talked about the players, the style of play and he was talking our language - he was talking Rotherham football.
“We had long meetings with him, he spoke with confidence and the board members who were present were very impressed with what he had to say, and what he could do for Rotherham United.
“I’m not looking for miracles, I’m looking for a guy that knows what he is doing and can get the systems working like we have seen in the past and start making statements in the Championship, but more importantly getting the points that we feel we’ve lost.”
Jackett has been out of work since leaving Wolves in the build-up to the new campaign and arrives in South Yorkshire with a wealth of experience, having taken charge of over 800 games.
He has managed in the Championship with Watford, Millwall and Wolves while also taking the latter two to promotion from League One - an achievement that may have been taken into consideration by the Millers board owing to their current plight.
Jackett’s appointment sees him become the fifth manager in the Millers hotseat since September 2015, following in the footsteps of Steve Evans, Neil Redfearn, Neil Warnock and Stubbs.
Stubbs was appointed on a three-year deal in June but became the club’s shortest-serving manager, getting fired after just 14 games, and he leaves with statistically the worst record in the club’s history with a win ratio of just seven per cent.
Jackett’s first game in charge will be Saturday’s Championship visit of Reading, where the Millers will hope to avoid equalling a club-record seven straight defeats in a single season.