In an announcement made on the club's official website, the board confirmed the ex-Watford player's appointment, with the new manager set to take charge of first team affairs following Tuesday evening's League One clash against his former club, Swansea City.
Explaining his decision, chairman John G. Berylson highlighted his reasons for appointing Jackett.
"Kenny was one of the first names on our list after we had drawn up our criteria for a new manager.
"Initially we thought we would not get our man, but we remained determined to try, and eventually after meeting with him on Tuesday afternoon, he agreed to join us.
"We were careful to take our time, and we did speak to a number of very good candidates, but Kenny is the one who came through as the strongest in the end and we were delighted when he accepted our offer.
"His management skills are first class, and I am confident he will soon have Millwall moving up the League."
"I'm excited to be joining Millwall and to be offered this challenge," added Jackett. "This is a club with tremendous potential and a passionate support and I am looking forward to meeting the players and getting underway later this week."