Footage on social media showed what looked like smoke shooting out from a pipe into the sky from the hospital building on Friday evening.
St George’s said in a statement that the boiler valve in the hospital’s day surgery unit had burst but that the escaping steam was “harmless”.
They added that it posed no risk to patients, staff or the public, before apologising to local residents for any inconvenience.
During the evening, people nearby took to social media to ask about a mysterious noise sounding throughout the area.
One person wrote: “Anyone else hearing a really loud rumbling noise in Tooting…?”
Another replied with footage of the incident, adding that it seemed to be a burst air vent.
A different Twitter user said: “Looks like a burst pipe on the roof of a building at St George’s Hospital.
“Pretty loud when you get up close. Been going on for about 45 mins now.
They later added: “Something probably unintentional going on round the back of St George’s Hospital, Tooting.
“Is it oxygen? Nitrogen? Water? Something very high pressured.”
Someone else said: “I live very near and can see that from my house. Saw three fire engines.”
While another Twitter user joked: “Think there’s a new pope of Tooting.”
In a statement, the hospital said: “This evening, a boiler valve in our day surgery unit at St George’s burst.
“The steam that escaped into the air was harmless, and posed no risk to patients, staff or the public.
“The problem has now been resolved, and we are sorry to local residents for any inconvenience caused.”