The group of around a dozen caravans and vehicles is understood to have arrived at Sandall Park on April 29, with the travellers believed to have illegally accessed the area.
South Yorkshire Police said: “Officers from the local policing team have visited Sandall Park and issued a notice to those staying on the site, directing them to leave as soon as reasonably practicable.”
Sandra Crabtree, chairman of Friends of Sandall Park said: “Parks are for recreation, exercise and play.
"They are not for traveller encampments.
"This is just so wrong in so many ways.
"Our volunteers put their heart and soul in to ensuring it is a fabulous place for families to enjoy outdoor space.
"It is a council owned park and we hope they resolve the situation quickly so it can once again be used for the purposes intended."