The girls, aged 14 and 15, were "inappropriately touched" near to the playground in Prospect Park between 16:15 and 16:40 GMT on Monday.
The offender assaulted the victims in two separate incidents before running off towards Waverley Road, police said.
He is described as a white man with tanned skin in his late twenties and about 6ft with brown hair which was longer on top and gelled.
Investigating officer, Det Sgt Stuart Streeter of Thames Valley Police, said: "He was wearing a grey hooded top, a dark coloured T-shirt, grey tracksuit bottoms with a black stripe down each leg and trainers.
"He was carrying a mobile phone that was orange or had an orange case.
"The park is popular with joggers, dog-walkers, children who use the playground facilities, and children and adults who use the sports facilities next to the playground."
Police will be carrying out additional patrols in the area following the incident.
Two teenage girls have been sexually assaulted at a park in Reading.