The Ghanaian youngster has long been in talks with Saints about a summer move to England and local sources say a deal is now struck.
Diario de Valladolid says Saints have met the €12m buyout clause in Salisu's deal. It's understood the defender has already passed a medical held in Spain.
Sants had been battling Rennes for the defender, though their president Nicolas Holveck explained all last week.
"Unfortunately, we knew that Mohammed preferred the Premier League because he is an English speaker. We respect his choice. There were doubts as to his ability to obtain a work permit in England. It seems he acquired it," declared Holveck.