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.
(Reporting by Steve Tongue, editing by Ed Osmond)
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