Son’s conversion was his 11th goal in the FA Cup since the start of the 2016-17 season — more than any other player.
Tottenham Hotspur rode their luck to reach the FA Cup fifth round as Son Heung-min's late penalty sealed a 3-2 victory in a rip-roaring replay against Southampton on Wednesday.
As in the 2-0 Premier League win against Manchester City at the weekend, Jose Mourinho's side were often outplayed but showed character to set up a home tie with Norwich City.
South Korean Son has often been the go-to player when lead striker Harry Kane is injured and again he stepped up to convert in the 87th minute after being taken down by keeper Angus Gunn in a frenetic finale.
Son's conversion was his 11th goal in the FA Cup since the start of the 2016-17 season — more than any other player.
It was tough on Southampton who deserved more, according to Tottenham manager Jose Mourinho.
"I have to be honest and say I think the best team lost on the pitch but my team were the ones with more heart and went to their limit," said the Portuguese, whose one FA Cup triumph came with Chelsea in 2007.
"So many difficulties to even build a team, the options were not options to change the game. My team deserved to win — but the best team lost."
Southampton played nearly all the football after unluckily falling behind in the 12th minute when Jack Stephens attempted to block Tanguy N'dombele's off-target shot but only deflected it past a helpless Gunn.
Ralph Hasenhuettl's side responded in a dynamic fashion with Danny Ings smashing a shot against the bar after 18 minutes.
They did level after 34 minutes when Shane Long netted from close range after Hugo Lloris parried the ball into his path after a smart turn and low shot by Nathan Redmond.
Southampton's James Ward-Prowse was carried off five minutes before halftime with a knee injury but they dominated possession after the break as Tottenham were reduced to playing on the counter-attack.
One of those should have seen Spurs reclaim the lead when Son crossed for Lucas Moura but he headed wide.
With both sides throwing caution to the wind in search of victory it was Southampton who appeared to be heading through when a superb forward foray by Redmond ended with Ings cutting in and slotting a clinical right-foot shot past Hugo Lloris.
Roared on by the home crowd Tottenham drew level when Moura danced through the Southampton defence and fired a low shot into the corner.
With extra time beckoning Dele Alli played in Son who dummied Gunn but tumbled under minimal contact.
A short VAR check confirmed the penalty and Son coolly dispatched the spot-kick.