The Brazilian playmaker will move from the Coppa Italia champions after four years at the capital club and will continue his career at San Siro
The Nerazzurri were desperate to strengthen their squad after a succession of disappointing results in the past few months and had already brought in Danilo D'Ambrosio from Torino during the January transfer window.
Hernanes had established himself as a firm favourite with Lazio fans following his arrival at the Stadio Olimpico from Sao Paulo in 2010, playing 154 matches for the Biancocelesti and scoring 40 goals.
However, he has now penned a contract with Inter until the summer of 2018, worth €2.8m per season.
The Brazilian insisted, though, during an interview with Lazio Style Radio that financial gain had had nothing to do with his decision to move to San Siro.
"I'm not leaving Lazio for money; my wife Erika knows the truth," he explained.
“Now I've made my choice, I would like to say thanks to all of you, the president, my team-mates and all the supporters.
“In my life I always tried to do my best, now I have the feeling it's time to make a change.
"It has been a hard night [Thursday], I didn't sleep at all. I knew the supporters loved me, I couldn't imagine they did so much. I'll keep them in my heart"
Inter currently sit fifth in the Serie A table with 33 points from 21 games.