The 28-year-old has signed on at San Siro for an undisclosed feeand will aim to boost Inter's flagging bid to qualify for the UEFA Champions League.
Walter Mazzarri's men are 11 points behind the third-placed Napoli after a poor run of form saw them lose their early-season momentum.
"F.C. Internazionale wishes Hernanes a very warm welcome to the club," an Inter statement read.
"The Brazilian midfielder's transfer from S.S. Lazio has now been completed and all the paperwork filed with the Lega Serie A."
Hernanes has made 24 appearances for Inter in all competitions this season, scoring four goals.
He spent three and a half years at Lazio after arriving from Sao Paulo in his native Brazil.
Hernanes is a Brazil regular and played in all five matches at last year's FIFA Confederations Cup, which they won.
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