Rain's agency Cube Entertainment stated that the person in the nude photo is not Rain and vowed stern legal action against those who first uploaded the photo along with the misleading comments.
The photo, claimed to include Rain's naked body, has circulated since Thursday along with the caption "Rain's naked photo released from Kim Tae-hee's lost phone" on online communities.
Kim Tae-hee is South Korean top-tier actress who has been dating Rain since last year.
In the photo, a man is having a shower with eyes covered, which makes it difficult to identify who the person is.
"There are quite so many people who mistake the person in the picture for Rain, so we decided to take legal action. His girlfriend Kim Tae-hee never lost her phone," said a lawyer representing Rain.