The Republic of Ireland international spent the 2021-22 campaign on loan at Fratton Park, where he finished second in The News/SportsMail player of the season poll.
Bazunu will become the 55th player to play in both blue and red on the south coast.
But how many of those names have featured for Pompey and Southampton in the 21st century?
We’ve taken a look at the 13 players who have appeared for both clubs since the turn of the century.
1. David Connolly
The ex-Republic of Ireland international spent three years at Southampton, where he scored 17 times in 67 outings. After spending six months without a club, Connolly returned to the south coast joining troubled Pompey in League One. During his two-year stay at Fratton Park, the striker netted 12 goals in 38 appearances. Photo: Pete Norton
2. Jhon Viafara
The former Colombian international started his career in England with Pompey after joining in July 2005. His move failed to hit the heights expected as he managed to make 15 outings during his season-long stay. Following a spell in Spain, the midfielder returned to the south coast, joining Southampton, where he spent two seasons. Viafara was jailed for 11-years in the USA in 2021 after being found guilty of trafficking £21m worth of cocaine into the United States from Colombia. Photo: Mike Hewitt
3. Martin Cranie
Cranie came through the ranks with the Saints and made 28 appearances, which included loan spells with Yeovil and Bournemouth. After departing the Reds in the summer of 2007, he linked up with former boss Harry Redknapp at Pompey. He made just five appearances though, two of which came in the Premier League. Photo: Julian Finney
4. Peter Crouch
The most recognisable name on the list first arrived in 2001 in a £1.5m move from QPR. However, he lasted just 10 months with the Blues before making a £5m switch to Aston Villa in March 2002. The ex-England international joined the Saints in 2004 and made 33 appearances, scoring 16 goals. Crouch returned to PO4 in July 2008 in a £9m deal from Liverpool and netted 16 times in 49 outings for Pompey. He departed for Spurs a year later. Photo: Christopher Lee