A nearly naked man nicknamed the "Barefoot Caveman" has stolen the show at the Boston Marathon after completing the event in just bare feet.

Donning only a tan-coloured loincloth, a bone necklace, and just the skin on his feet, 50-year-old John Glen Raines finished the gruelling 42km race in 3 hours, 40 minutes and 43 seconds.

Raines' antics lit up social media when he was photographed crossing the finish line, fist-bumping a fellow runner while in his precariously skimpy outfit.

Despite the presence of wind, the "Barefoot Caveman's" kit remained intact.

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It's not Raines' first time donning the loincloth after running his first marathon in the prehistoric getup in 2010.

The 50-year-old is an experienced marathon runner, with records of his feats dating back to 1999.