The 21-year-old came through the youth ranks at Easter Road, making 27 appearances and scoring two goals before his release last summer.
Following a successful trial period with the Paisley side, he subsequently signed a one-year deal with the option of a second year.
But after struggling for regular game time so far this season, the hitman departed Saints this morning, terminating his contract by mutual consent.
As exclusively revealed by the Tele, Ton boss Jim Duffy had been monitoring Caldwell’s situation with interest.
And he today beat off interest from other clubs, believed to include League One rivals Dunfermline, to secure the forward’s signature and bolster his depleted frontline.
Morton have also today confimed the signing of goalkeeper Grant Adam until the summer.
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