The 25-year-old was named the Saints’ Player of the Year in 2018-19 and is now tied to the club until the summer of 2023.
He will be hoping to kick on from this point, having taken his overall record for Southampton to 18 goals in 129 appearances.
Redmond’s efforts to date have earned him one senior England cap, to go with the 38 he collected at U21 level, and he was included in Gareth Southgate’s provisional UEFA Nations League finals squad.
Another strong showing this term could earn him a place in a European Championship party, with his club future having been tied down with the signing of fresh terms.
Redmond told the Saints’ official website on that deal: “I’m really happy to have signed, and I'm just wanting to get the season up and going now.
“We've got a great group of players here, who I've been with for the last three years. I see them every single day and it's a pleasure to go out there and perform with them at the weekend and train hard with them every single week.
“I feel like we can hopefully build something special and to be a part of it for the next four years is good for me.”
He added: “I feel I've developed a lot as a player, but not just as a player, but as a person as well.
“It's been a crucial part and another step in my development. I've had ups and I've had downs with the club, but it's been a constant learning process. Looking back, I've thoroughly enjoyed every single moment.
“I'm getting older and it's another stage in my development. I came here as a young lad with a fair amount of games and experience under my belt, but of course there was another level when I came here, which I had to step up to with the players who were here at the time.
“The players who have come and gone, and the players still here now, I feel like I have grown with them the last couple of years and it's a good feeling to now know I'm crossing over that border into a more senior player now, and that comes with more responsibility.
“It's not always something I've wanted in the past, but I think now I'm definitely ready for it.”
Southampton boss Ralph Hasenhuttl said of Redmond’s agreement: “It is very good news for the club.
“He's a player who showed in the last season that he is very important for Southampton and I think it's a massive signal for the club, and for us all, that he wants to follow our way. It's a very positive day for us.”