The midfielder will begin a two-year scholarship on Wednesday 1 July, and will turn pro on his 17th birthday in February.
Slattery was a regular for the Young Lions under Steve Cooper during the 2014-15 campaign.
He skippered the side to success in France as they won the Montaigu Tournament with a 3-1 win over the hosts in the final back in April.
He made 25 appearances for the Saints’ U18s last term, netting a spectacular strike in their FA Youth Cup win over West Ham.
He also made his U21s debut for the club while still only 15.
England U16s players celebrate Montaigu Tournament glory
“I’ve progressed over the years and that is because of Southampton,” Slattery told SaintsFC.co.uk.
“From being eight years old all I’ve wanted to do is play for the first-team and get a professional contract.
“I’m delighted so much to get that contract now.”
Les Reed, executive director of football at Southampton, added: “This is a fantastic opportunity for Callum to follow in the footsteps of a long line of homegrown players at this club.
“Callum has demonstrated the desire and talent to strive for first-team football at Southampton.
"He knows this will require the same level of hard work and dedication previous demonstrated by the likes of Gareth Bale and Alex Oxlade-Chamberlain, and more recently by fellow midfielders James Ward-Prowse and Harrison Reed.”
Slattery scored a stunner against West Ham in The FA Youth Cup
He continued: “Callum has the correct profile to follow the pathway to the first team we have created.
“He has achieved international recognition, and will now join the professional ranks motivated to achieve his dream of playing for Southampton's first team and England.”
Slattery will be hoping to make an early impression at the beginning of next season – not only to impress his club coaches.
He will be hoping to win a place in the England U17 squad for the St. George’s Park International Tournament at the end of August as the Young Lions prepare for the European Championship qualifiers,
