Jackett was appointed as Alan Stubbs’ successor on October 21, but after taking just one point from his first five games he has decided to walk away.
That leaves the Millers, who are 11 points from safety in the Championship and set for a return to League One, looking for a third manager of the season and a sixth in 14 months.
A club statement read: “Following the game against Leeds United at the weekend, Kenny Jackett tendered his resignation and accordingly, his employment with the club has terminated with immediate effect.
“Kenny’s resignation came as a shock to the club and is disappointing. The club will now focus on making appropriate arrangements for the first team.”
Former Coventry and West Brom manager Tony Mowbray has been heavily linked with the job.