The reclusive star has spent the entire summer holed up in his Los Angeles mansion, but emerged yesterday wearing the outfit, and a pair of sunglasses, which he had purchased from a local fancy dress store.
The shop manager said: "He spent a long time trying various costumes before he chose the gorilla suit and bought it for $60."
Robbie - who recently split from long-term girlfriend Ayda Field - insisted on changing into the bizarre outfit before leaving the shop, and then strolled around Hollywood holding hands with a mystery woman.
The 'Angels' singer has become increasingly obsessed with aliens in recent months, and friends are said to fear for his mental state.
Ex-girlfriend Tara Palmer-Tomkinson said: "Of course I'm worried for Robbie. He's a genius and when you're that smart and have been in the public eye for so long you want to escape."
It is believed the 34-year-old star's fascination with UFOs was a major factor in his split with Ayda.
The pair's relationship allegedly hit the rocks because the 29-year-old actress wanted a partying lifestyle, whereas Robbie is only interested in spotting UFOs and playing golf.
Only last week, Robbie was identified as a prime alien abduction target by extraterrestrial expert Michael C. Luckman.
He said: "Robbie is now becoming a point man for contact with extraterrestrials. Robbie could easily disappear and then come back as an ambassador for their race."
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