Police were called to Castle Road at 10.20am after concerns for the man’s welfare were raised.
The force helicopter was tasked to help search the area, and units continue to carry out searches and enquiries on the ground.
The helicopter has now left the scene but enquiries remain ongoing, a force spokeswoman said.
She said: “Police were called at 10.20am today to Castle Road, Dartmouth following reports of concern for the welfare of a man in his 20s who is thought to be missing.
“Units including the police helicopter have attended the area are carrying out searches and enquiries in order to locate the man.
“Enquires remain ongoing at the moment.”
The helicopter completed an initial search and left the area at around 1.15pm, the spokeswoman said.