Greene was in the U.K. capital to promote The Twilight Saga: Eclipse earlier this month (Jul10), while Jonas visited the city to support his brother Nick's starring role in West End musical Les Miserables.
And the young stars went for a romantic dinner date at The Ivy, according to Britain's News of the World.
A source tells the publication, "They had a very romantic night at The Ivy. They arrived separately and then left out (sic) by the back door where there were two vans with blacked-out windows waiting to whisk them away.
"They looked like a lovely couple. They had a few drinks and were clearly enjoying not being hounded like they would have been in Los Angeles."
Greene has previously been linked to Brock Kelly and Adrian Grenier, while Jonas recently split from Disney star Selena Gomez.