Mrs May described the attack on the 15-year-old refugee as “absolutely terrible” and said her thoughts were with him.
Footage of the boy being attacked and threatened with drowning at his school in Huddersfield provoked a flood of sympathy after being shared on social media.
An online appeal set up to help the family topped £130,000 in 24 hours and football stars from Huddersfield Town invited the boy to be their guest at a Premier League game.
Asked about the incident during her visit to the G20 summit in Argentina, Mrs May said: “I thought it was absolutely terrible ... what (he) went through. Our thoughts are with him.
“But I think if you look at what happened, the real spirit of Britain came in the response of the British people to that incident.
“As you say, most people were sickened and angered by it. The huge response of support ... that we have seen from the British people shows our true spirit and shows we are a welcoming people.”
The teenager thanked well-wishers for their support after arriving at Almondbury Community School with his father on Thursday.
His father said he did not want to give interviews to waiting media but, as they left, the teenager just said “thanks for everything” before getting into a waiting car.
The emergence of the video was followed by further footage of what was said to be the boy’s sister being physically abused at the same school.
A 16-year-old boy has been interviewed over the attack on the boy, and reported for summons for an offence of assault ahead of a youth court appearance.
The boy was questioned by police over the footage in which the 15-year-old victim, with his arm in a cast, is thrown to the ground.
He is dragged to the floor by his neck before his attacker says, “I’ll drown you”, while forcing water from a bottle into his mouth.
The teenage victim told ITV News on Tuesday: “I woke up at night and just started crying about this problem. They think I’m different, different from them.
“I don’t feel safe at school. Sometimes I say to my dad, ‘I don’t want to go to school any more’.
“I was just crying and I didn’t do nothing because I respect the school rules.”