Robert Stevenson, 65, of Portsoy, Aberdeenshire, was last seen driving his car on the B9031 at Dubford Crossroads near Gardenstown at around 4pm on Thursday.
He was then thought to have been travelling east in the direction of Pennan.
Police Scotland have asked for members of the public travelling on the road at this time to contact them.
They are also appealed to the public to check their outbuildings for his car and have asked drivers with dashboard cameras to check footage.
Mr Stevenson is described as white, 5ft 4in tall, of slim build, with brown eyes, short grey hair and a moustache.
He is believed to be wearing a white, pink and blue checked shirt, black trousers and shoes.
His taxi, a blue Dacia Duster, has AA Taxis on the roof sign and logos on both doors.
Inspector Megan Heathershaw said: “Our inquiries have been supported by HM Coastguard, the Police Scotland helicopter and police search teams however we have still not traced Robert and as time goes by we are increasingly concerned for his welfare.
“We would also ask local residents to check around outbuildings for his car, particularly more isolated buildings and similarly if you see Robert please contact us on 101.
“We would also appeal to local Hotels, Bed & Breakfast and Guest House premises to contact us if they have seen Robert or his taxi, especially in the last 72 hours.”