They believe the Nirvana frontman – who was found dead at his home in 1994 aged 27 – is performing under a fake name in Peru.
Footage of a singer called Ramiro Saavedra performing the band’s hit Come as You Are on a Peruvian TV show has gone viral this week.
Saavedra bears an uncanny resemblance to the late rocker, sparking speculation Cobain is making a living as an impersonator of himself.
One stunned fan wrote “OMG Kurt Cobain from Nirvana is alive!” while another simply said “He is alive!”
Cobain’s body was discovered at his US home in April 1994, with a coroner ruling he died of a self-inflicted gunshot wound.
But a flurry of conspiracy theories have emerged in the years since.
Fans of Tupac Shakur sparked a similar furore last month after claiming the rapper faked his death 20 years ago and is living in Cuba.
They shared a selfie apparently showing the star – best known for hit California Love – alive and well on the Caribbean island.
They also claim he is rapping under the name Kasinova tha Don.
A representative for Nirvana last night denied the claims about Cobain.
He pointed to the fact Saavedra plays the guitar with his right hand in the footage, while Cobain was left-handed.
He joked: “It’s true, Kurt is alive. He needed time to learn to play the guitar with his right hand. Finding left-handed guitars isn’t easy.”