Steven Tiffin, from Buckingham, was last seen in Century Avenue, Oldbrook, Milton Keynes, at 12.30am yesterday (Sunday).
He is white, 5ft 10ins, of slim build with short mousey brown hair and was last seen by a relative wearing jeans and a grey hooded top.
Steven has links to Aylesbury, Milton Keynes, Oxford and Northants.
He first went missing on May 19 but was found safe and well on the same day. Steven then disappeared for a second time on June 4 before being found the next day.
Sergeant Lou Heffernan said: “I am appealing for anyone who has seen Steven or anyone matching his description to contact police.
“If you see this appeal Steven please call 101 and let us know you are safe.”
If you have any information about Steven’s whereabouts, call 101 quoting occurrence number 43140092907.