He scored 22 goals in 33 league games for Torino this term under a co-ownership deal with Juve.
"In Ciro Immobile we have a flexible and dynamic forward who has been impressive in one of Europe's top leagues," Dortmund sports director Michael Zorc said in a statement.
"He fits with his outstanding qualities to our profile demand."
Coach Juergen Klopp has had to replace several squad members in recent seasons as domestic and European success has seen players like Shinji Kagawa, Nuri Sahin and Mario Goetze snapped up by bigger clubs.
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- Bayern Munich
- Michael Zorc