The iris-scanner can be used to unlock the phone simply by looking at it, which Samsung says provides "airtight security".
But researchers at Chaos Computer Club had easily tricked the device with a picture of an eye, Motherboard said.
Samsung told the BBC it was "aware of the issue".
The researchers first set up the phone's security by registering a volunteer's eyes using the S8 iris scanner.
They then took a photograph of one of the volunteer's eyes, using a digital camera with an infra-red night vision setting.
After printing the image, the researchers placed a contact lens over the photograph.
The team posted a video showing the S8 smartphone unlocking itself when it saw the false eye.
Samsung said its iris-scanning technology had been through "rigorous testing"  to "prevent attempts to compromise its security".
"If there is a potential vulnerability or the advent of a new method that challenges our efforts to ensure security at any time, we will respond as quickly as possible to resolve the issue."
Security expert Ken Munro said the discovery was "another reminder that biometrics is not a silver bullet".
"Personally, I prefer fingerprints to iris unlock. Your fingers are already holding your phone, so why not use prints rather than wave your phone in front of your face?
"If you want to be really secure, choose fingerprints and a secret number. If you must have iris unlock, please walk everywhere with your eyes closed, so your iris can't be photographed."
Galaxy S8 owners have the option of using a password or secret number to unlock their phone, instead of using the iris scanner.
Samsung's eye-scanning security technology, used on the new Galaxy S8 smartphone, has been fooled with a photograph and a contact lens.