Saints boss Nigel Adkins told the club website: "We're delighted to be able to secure him on a long-term deal.
"He has a good partnership growing with Rickie Lambert, but more importantly he's part of the team that we've got."
Do Prado, who has scored eight goals in 22 appearances for the Saints, added: "I'd like more games and more goals. The aim is for promotion this season and then next.
"The football and the fans make me very happy here. I'm very happy with the new contract and I'm enjoying working with the players and the manager."
Source: BBC Sport
Source: BBC Sport