Motherwell announced the sale of the 21-year-old in a tweet which stated the player had moved for "a club-record fee". Though the exact sum has not been confirmed, it must therefore be more than the £1.75million the Fir Park side received from Celtic for Phil O'Donnell in 1994.
Turnbull was originally set to sign with the Scottish champions last summer - only for it to fall apart at the last minute. A medical at Celtic found a serious issue with his knee which required surgery.
Motherwell retained their star asset for the 2019/20 campaign though he was only able to appear twice from the substitutes bench before the season was curtailed by the coronavirus pandemic. He has started all five league games for Stephen Robinson's men since the restart.
The playmaker agreed to a one-year extension to his contract in March this year as he sought to thank Motherwell for standing by him during his injury rehabilitation.
He has now put pen to paper on a four-year deal to realise his ambition of signing for Neil Lennon's men.
A Celtic statement read: "Celtic Football Club are delighted to announce the signing of David Turnbull, who has joined the club on a four-year deal from Motherwell.
"The highly-rated midfielder came through the ranks at Fir Park, making his debut back in February 2018. He has since gone on to make 41 appearances for the club, scoring 16 goals, the most recent coming in a 2-2 draw against Livingston earlier this month.
"The 21-year-old had been linked with a move to Celtic last year, but that did not transpire, with the player undergoing a knee operation before working his way back to full fitness.
"Now the move to the Scottish champions has been confirmed, and the new Celt is delighted to have joined Neil Lennon’s side."
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