The 20-year-old has agreed a four-year contract subject to the relevant paperwork being completed on July 1.
Djenepo scored 11 times for Standard Liege last season, helping them finish third in the Belgian top flight and qualify for the Europa League.
"We are very happy to welcome Moussa as the first new player to join our team ahead of the upcoming season," Saints manager Ralph Hasenhuettl said following the announcement.
"He is an exciting player, with tremendous pace and good finishing abilities, who we have watched closely for some time."
Djenepo will link up with his new team-mates for pre-season after representing Mali at this summer's Africa Cup of Nations.
"It's a good club that is progressing, it has great facilities, and it's a place that can help me to really kick on and progress," said Djenepo.
"I am looking forward to it and I want to get to know my new family. I can’t wait to be around my new teammates and work hard alongside them."