Saints have agreed a four-year contract with the 20-year-old Mali international, subject to paperwork being completed on July 1.
He will become Southampton’s first signing of the summer, and also the first arrival since Ralph Hasenhuttl was appointed manager last December.
Football's top transfers from the 2019 summer transfer window so far
Djenepo scored 11 goals as he helped Liege finish third in the Belgium top division and qualify for the Europa League in 2018/19.
Hasenhuttl told the Southampton website: “We are very happy to welcome Moussa as the first new player to join our team ahead of the upcoming season.
“He is an exciting player, with tremendous pace and good finishing abilities, who we have watched closely for some time.”
The winger will join up with his new Southampton team-mates following the Africa Cup of Nations.
“It is something I am very proud of,” Djenepo added. “It’s a dream come true. Southampton is a great club and I am very happy.
Click Here: highlanders rugby gear world