Police said the man, aged in his 20s, was discovered early on Saturday morning on the Upper Knockdreda Road in south Belfast.
He was found lying close to the junction with the Cregagh Road at about 1.30am and had serious head injuries.
The ambulance service had been contacted about the discovery.
Police said they had a report that the man got out of a taxi close to the Braniel Estate and appealed for anyone who may have seen him on foot in the area between 12.30am and 1.30am.
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