Van Wyk's impending exit from Franklin's Gardens was confirmed by the club on Thursday evening.
And the South African, who arrived at the Gardens from Western Force in 2017, will now be switching to Saints' local rivals.
Few players have moved between Saints and Tigers, though Ryan Lamb did swap Northampton for Leicester back in the summer of 2013.
And van Wyk will now do likewise, having never been truly able to cement the starting spot at loosehead, despite some impressive showings.
The 29-year-old has so far made 83 appearances for the black, green and gold.
He will stay at the club for the remainder of the season, competing for starts with the likes of Alex Waller, Nick Auterac and youngster Manny Iyogun.
Saints also have Danny Hobbs-Awoyemi as a loosehead option but he is currently sidelined with an Achilles injury.
Van Wyk said: "I feel very fortunate to have had the opportunity to play for Saints these last four years.
“I want to thank everyone who has been a part of my time at Franklin’s Gardens, and I will be completely committed to helping the squad for the remainder of my time at the club.”
Saints wished van Wyk well on their website.
The club statement said: "Northampton Saints would like to thank Francois and his wife, Andi, for their contribution to the club over the last four years."