The 66-year-old returned for a second stint in charge at Shrewsbury in June 2010 and guided the club to promotion from League Two in 2012.
Turner successfully kept the club in the third tier of English football last season, but has now stepped down after a 3-0 home defeat against Rotherham on Saturday left Shrewsbury 20th in the table with one win from their last eight matches.
"It's a very sad decision. It's a club that I've really enjoyed working at; it's a club that gave me my first opportunity in management so I'll always appreciate that and will always be close to the club," he told Shrewsbury's official website.
"I just felt it wasn't necessarily just Saturday. I think there's been a sequence of results where for several weeks now I thought I could improve things.
"I thought I was the right man to get the results going and it hasn't materialised so I felt that there was a need for a decision to be made."
The former Aston Villa, Wolves and Hereford United boss concedes his 36-year managerial career is likely to have reached its end.
Turner added: "I've got to be honest. You sit and contemplate the future, and I would think it would be fair to say that my managerial days are over.
"There might be something I can contribute somewhere. There may be a door open to come back at sometime in the long-term future here (at Shrewsbury)."
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