The industrious midfielder moves for an undisclosed fee from Scottish Premier League side Glasgow Celtic, where he won the title in 2011/12 and 2012/13 as well as last season's Scottish Cup.
Wanyama was crowned as the SPL Young Player of the Year at the end of last season, during which he made 49 appearances and scored nine goals.
One of those goals came in the Hoops' memorable win over Spanish giants FC Barcelona in the UEFA Champions League last November.
Wanyama's signing is the latest demonstration of the Club's plan to recruit young and talented players on long-term contracts, and follows on from the arrival of Croatia international Dejan Lovren last month.
"It feels great and I am happy to be here," Wanyama said immediately after putting pen-to-paper at St Mary's this evening.
"Finally it has happened and we've come to the end [of negotiations], so I am delighted. There were other options, but Southampton is a club with ambitions so I chose to came here.
"Southampton is a good club and it has good players, so I just want to train hard and work hard to be a part of the starting XI. We'll see where that will take me.
"I'm very grateful to be here and I'm looking forward to playing in the Premier League.
"I've seen that the stadium is great and the dressing room is looking good, and I know some of the players. I hope they will help me and I think I will get along with the others, so it will be good.
"I am happy to be a part of Southampton's squad. It means a lot because because it has been my dream to play in the Premier League and now I am here."
The first image of Victor Wanyama in a Saints shirt
First Team Manager Mauricio Pochettino was on hand to watch as his latest acquisition completed his transfer, and quickly expressed his happiness with the deal.
He said: "I am very pleased to have secured the signing of Victor because we are a big admirer of his talents and he will fit in well to the team here at Southampton.
"I want to stress that the deal could have been done sooner, but this shows that we do business on our terms. The most important thing that ourselves, Celtic and the player are all happy with the way it has all gone.
"The fact that a lot of top European clubs were also keen to sign him shows what an attractive prospect Southampton Football Club is to a player who had his pick of the teams in England and abroad.
"I've watched him perform at the top level in the Champions League, and I believe that he will be a valuable addition to our Premier League squad for the season ahead."
In Profile – Victor Wanyama
Victor Wanyama was born on 25th June 1991 in Nairobi, the capital of his native Kenya.
The son of former Kenya international Noah Wanyama, Victor started out as an attacking midfielder while playing for Kamukunji High School, where he broke into the first team at only 13.
During his teens, Wanyama spent time with the JMJ Youth Academy, Nairobi City Stars and AFC Leopards, the club with which his father made his name.
In 2007 he moved to Sweden with Helsingborgs IF. It was there that Victor linked up with his brother McDonald Mariga, who had been in Scandinavia since joining Enköpings SK in 2005 before climbing two divisions to move to Helsingborgs.
Wanyama also made his international debut in May of that year, appearing for Kenya in a friendly against Nigeria in his home city of Nairobi.
However, after Victor’s arrival, the brothers were quickly broken up when Mariga moved to Serie A side Parma on loan in August 2007.
Still only a teenager, Wanyama failed to settle in Helsingborg and returned to Kenya but was soon on the move again, linking up with Belgian club Germinal Beerschot on trial before signing a four-year deal to move there on a permanent basis.
He made his debut for Beerschot at the age of 17, playing in a goalless draw with KV Kortrijk on 4th October 2008. While that was his only taste of first-team action in 2008/09, Victor began to establish himself with the Kenyan national team, appearing in a friendly against Iran and a 2010 FIFA World Cup qualifier versus Tunisia in March 2009.
It was in 2009/10 that Wanyama came to the fore at Beerschot. He made a total of 20 league appearances plus a further one in the Belgian cup, while also continuing his rise to prominence with his country as they looked to qualify for the upcoming World Cup finals.
In the summer of 2010, Beerschot resisted overtures for Victor from Glasgow Celtic and CSKA Moscow, and they were rewarded with an excellent season from the midfielder as he made 34 appearances to help his side to a mid-table finish.
Wanyama scored his first professional goal in December 2010, notching the equaliser in a 1-1 draw at Westerlo. His second would arrive against the same opponents later in the season, with his 88th-minute strike rounding off a 3-3 draw the following April.
His first international goal would also arrive in 2010/11 as he scored an injury-time winner in Kenya’s 2-1 victory over Angola in a 2012 Africa Cup of Nations qualifier on 26th March 2011.
Celtic rekindled their interest in Wanyama at the end of the season, and this time they got their man as the midfielder completed his move to Glasgow on 9th July 2011.
He made his Celtic debut four games into the Scottish Premier League season, playing in a surprise 1-0 home defeat at the hands of St Johnstone. His second appearance was a more positive experience, though, as his side ran out 4-0 winners against Motherwell.
Wanyama’s first goal for the Hoops arrived in December 2011 as he scored in a 1-0 victory over Hearts, and he went on to be handed the SPL Young Player of the Month award a few weeks later.
He ended the season with an SPL winner’s medal as Celtic pipped a Rangers side that included current Saints midfielder Steven Davis to the title. In 2011/12 Wanyama played for his new club 42 times – five of which were in the UEFA Europa League – and with four league goals under his belt.
Celtic landed the SPL title again a year later, also winning the Scottish Cup when they beat Hibernian in the final last month – a game Wanyama missed through suspension.
The midfielder played for Celtic 49 times last season, scoring nine goals. The most memorable of those strikes came in a UEFA Champions League game against Barcelona as he netted the opener in a famous 2-1 win at Parkhead.
Victor ended the campaign as the 2013 Clydesdale Bank Young Player of the Year and with a reputation as one of the most exciting young midfielders in Britain, leading to interest from clubs at the top of the Premier League and across Europe.
He leaves Glasgow having played for the Hoops 91 times and with 13 goals to his name. Furthermore, while on international duty earlier this month, he was handed the armband by Kenya coach Adel Amrouche.