The incident happened at about 3.30pm on Tuesday near Blackburn, West Lothian.
The farmer was assaulted by three people, who then stole the keys to his green Honda 420cc quad bike and made off with it.
The victim was not seriously injured but police described it as a “despicable attack”.
The suspects are male, white and about 18 or 19, with local accents.
One was around 6ft, thin and was wearing a dark hooded top and bottoms with the hood pulled tight around his face.
The second man was about 5ft 10ins, stocky and tanned. He was wearing a black tracksuit with a hood up and around his face.
The third suspect was about 5ft 11ins and thin. He was wearing a light grey tracksuit and had a balaclava covering his nose and mouth.
All three were said to be covered in mud.
Pc Craig Ireland said: “This was a despicable attack on a farmer going about his business for the purpose of stealing his quad bike.
“Thankfully he was not seriously injured in this robbery but he has been deprived of a vehicle he needs to do the day-to-day work on the farm. We are keen to speak to anyone with information that can assist us.”