The victim was queuing for food at the branch in St George's Street, Canterbury at about 02:15 GMT on Friday when the assault occurred.
Investigating officers said three men entered the restaurant and began being noisy and bumping into people.
It is believed one of the group then set light to the woman's hair.
Officers have released CCTV images of three men they are keen to speak to regarding the attack.
Det Sgt Barry Carr said: "Fortunately the fire was put out quickly and the victim was not seriously hurt, but things could clearly have turned out much worse.
"This was a nasty and extremely dangerous thing to do, and I urge anyone who recognises the men in the CCTV images to contact me as soon as possible."
A woman had her hair set alight in a "nasty and extremely dangerous" attack at a McDonald's restaurant in Kent, police said.