Stuart Wilkin slotted the Saints into a fourth-minute lead at their Staplewood training ground but Samuel Shashoua equalised, before a double from Ryan Loft later in the first half sealed the victory.
There was a first Barclays Under-18 Premier League start for centre-half Joy Mukena in the south coast sunshine, while striker Aramide Oteh stepped off the bench in the second period for his first league appearance of the season.
It was the Saints who started quickest, though, and, when Olufela Olomola's effort was blocked, Wilkin was there to fire past goalkeeper Tom McDermott to give the home side the lead just four minutes in.
Tyreke Johnson’s shot caused problems for McDermott shortly afterwards but Olomola couldn't make the most of the follow-up in a chance Southampton would come to rue.
We soon got ourselves into our attacking rhythm and on 12 minutes we equalised as Shashoua exchanged passes with Zenon Stylianides before smashing past Saints stopper Jake Hallett – his second goal of the week after also hitting the target in Tuesday's Under-20s friendly at Barnet.
Marcus Edwards and Shashoua both tested Hallett's resolve as the half went on, but it was a defensive mistake by the Saints that led to us taking a 31st-minute lead, Loft nipping in to make it 2-1.
We had to stay alert at the other end, though, and it was only a goal-line clearance from Mukena that denied Olomola an equaliser, but a third goal for us two minutes into first half stoppage time made our position more comfortable.
Edwards won possession in midfield and found Shashoua, who in turn set up Loft to double his tally for the day.
Keeper McDermott had a busy start to the second period, denying Harley Willard on 55 minutes and substitute Carel Kayembe shortly afterwards, before keeping out Joe Lea's shot from distance.
At the other end, Stylianides tried a shot but could only find the roof of the net, with Shashoua then testing Hallett with a free-kick.
The Saints were forced to play out the closing stages with 10 men because of an injury with all three allowed substitutes already deployed and it was ourselves who had the final chance of note shortly before the end as Joe Muscatt got forward and drove a shot towards goal, but Hallett tipped it onto the bar and behind.
Spurs U18s: McDermott, Hayford (Paul 73), Muscatt, Owens, Mukena, Brown, Edwards (Bennetts 53), Stylianides, Loft, Shashoua, Daly (Oteh 68). Substitute (not used): Whiteman (GK).