The Norway international had joined them from another German side, Borussia Moenchengladbach, a year ago on a free transfer.
Nordtveit played 21 games in all competitions for the London club.
He will get the opportunity to play Champions League football after Hoffenheim achieved their highest-ever league position of fourth last season.
"He is an excellent professional and we wish him all the very best at Hoffenheim," West Ham manager Slaven Bilic said.
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(Reporting by Steve Tongue, editing by Ed Osmond)
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