The 20-year-old Saints academy graduate has become a regular under Ralph Hasenhuttl and has made 16 Premier League appearances so far for a total of 30 in all competitions this term, according to Transfermarkt.
Valery joined from French club Rennes in 2015 and progressed through the ranks at the south coast outfit, making his full debut in Mark Hughes's last match in charge, a 2-2 draw against Manchester United in December.
The right-back has progressed and improved over the past few months, scoring in his last two matches against Manchester United and Tottenham Hotspur respectively, the latter of which prompted a late comeback to claim a 2-1 victory over Mauricio Pochettino's side.
After Southampton confirmed the news with an article and Twitter post, Valery himself tweeted a message expressing his delight at committing himself to the club:
Proud day for me and my family today. Delighted to have signed a new long-term deal at this great club @SouthamptonFC . Looking forward to the up and coming years. Ready to express myself and show the fans what I can do pic.twitter.com/NuFERAKihW— Yan Valery (@yan_valery) 14 March 2019
Some Saints fans responded to him with congratulations and optimism regarding how he will develop further at St Mary's, including one who compared him to the legendary Lilian Thuram:
Your family and #94 will be very proud of you mate #saintsfc— Robbie (@EditSouthampton) 14 March 2019
Well deserved Yan! Look forward to many more years of you in a shirt!— Ben Stanfield (@BenStanners) 14 March 2019
Good work Yan dawg, become a legend !— Yoshida’s Story (@RalphHasenhuttl) 14 March 2019
Good man Yan,think we could have the next Lillian Thuram in our hands— Luke Fletcher (@Superfletch81) 14 March 2019
The club is lucky to have you mate. Good luck.— Nigel Murdoch (@NigelMurdoch1) 14 March 2019