The man hid himself in the rear wheel compartment of the plane which landed at Heathrow Airport on Sunday. He was taken into police custody in London but later released without charge.
He had bruises and hypothermia from outside temperatures as low as -41C, Austrian media reported.
He survived because the plane flew at a low altitude to avoid stormy weather.
The man apparently got under a fence at Schwechat airport in Vienna and climbed into the undercarriage of the first plane he saw without knowing its destination.
The plane belonged to a sheikh from the United Arab Emirates and had been standing empty on the tarmac at Schwechat airport since Thursday.
It flew without passengers to Heathrow, where the Romanian was picked up by police and arrested for stowing away.
He could have been charged or fined or given a fixed penalty, the Metropolitan Police told the BBC. But he was cautioned and freed with no further action being taken, PA news agency reported.
The man could also have been handed to the UK Border Agency.
But it is understood that there is no immigration issue and that the agency will not seek to deport him, according to PA.
As Romania is part of the EU, the man is free to enter the UK.
A spokesman for the Civil Aviation Authority (CAA) said the stowaway was "very lucky" to be alive.
"If they don't find the right part to stow away, they can be crushed when the undercarriage comes up," he said.
He added: "Because of the altitude and temperatures during the flight, there is a severe risk to them through exposure and lack of oxygen.
"If that doesn't kill them, then they could be unconscious when the aircraft descends, and that can mean that when the undercarriage opens again, they will fall out."
According to Austrian media reports, the man just wanted to get out of Vienna and look for work.
Romania is a member of the European Union, so Romanians can travel to the UK for holidays. However, controls on Romanians working in Britain remain in place.

With characters like Sherlock Holmes and Dr Strange under his belt, he's signed up to play a supposed 41-year-old who is really 400.
He'll star as Tom Hazard in the film adaptation of the forthcoming How to Stop Time by author Matt Haig.
Described as a "wildly imagined love story", it follows Haig's children's books such as A Boy Called Christmas.
How to Stop Time won't hit shelves until July, but we know Hazard's youthful looks are down to an "extremely rare condition".
"The prospect of Benedict Cumberbatch playing Tom Hazard is a hugely exciting one and I could not be happier," Haig said.
The writer is best known for his quirky - and dark - stories about family life.
Cumberbatch's production company SunnyMarch will make the film with Studiocanal, while the actor will also take on the role of executive producer.
Before that, he'll be coming back as the eccentric surgeon Dr Stephen Strange in the Marvel superhero movies Thor: Ragnarok and  Avengers: Infinity War.
He'll also star in The Current War as the real-life genius inventor Thomas Edison.
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A 20-year-old Romanian man had a lucky escape after stowing away on a plane flying from Vienna to London.
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