The 32-year-old was spotted smoking on a street corner with fingernails painted black when an on-looker thought him to be a beggar, reported Sun Online.
"I didn't recognise him at first - I thought it was some homeless guy who was scrounging a smoke. We had a brief chat with him and his accent was American. It was only then that we realised he was Macaulay Culkin," said an on-looker.
"We chatted for about five minutes and he said he was in London seeing friends though he was vacant, speaking very quietly and mellow. He looked spaced out. You'd have thought he was a down-and-out, not a famous film star," he added.
He married actress Rachel Miner in 1998, divorced four years later and went out with Mila Kunis for eight years before they split in 2010.
He was arrested in the US in 2004 for possession of marijuana and Alprazolam and Clonazepam anti-depressants and given three one-year suspended jail sentences.
Last year Macaulay denied American magazine reports claiming he was hooked on heroin.