The 19-year-old will immediately be loaned back to the French club and will spend the 2014/15 season in Ligue 1 before moving back to Anfield.
"I am very, very happy and delighted a club as big as Liverpool showed interest in me. I'm very excited," said Origi.
"I know it's a club with a great history, great fans and a lot of great players. For me, Liverpool is one of the greatest clubs in the world and I'm very excited to be part of this great history."
Origi scored just six goals in 35 appearances for Lille last season but was mainly used as a wide man.
However, he impressed at the World Cup in a central striker position and scored the winner in Belgium's 1-0 win over Russia.
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- Sports & Recreation
- Rickie Lambert