Roundhay Environmental Action Project (REAP) runs the monthly Oakwood Farmers Market, held at Oakwood Clock.
It had displayed banners publicising the not-for-profit event, staffed by volunteers, at a series of locations nearby in the week before each monthly market.
But organisers have said that four of its banners have been stolen from the Oakwood Clock and the fence bordering Roundhay School.
Pat Urry, chairwoman of REAP's market committee, said: "We have permission to display banners at various locations in the week before each market.
"Recently four of our banners have been taken from the Clock itself, and also from the Roundhay School fence."
The market celebrated its 10th anniversary in May this year.
Mrs Urry said it was set up to boost the local economy by attracting visitors to Oakwood, promote healthy eating and increase community spirit.
However, as each banner costs Â£30, she warned that their "loss is not sustainable".
She said the group is now working with local police.
The farmers' markets in Oakwood run on the third Saturday of every month.
The next market takes place on Saturday, November 17.