Two teenage girls contacted police after seeing a man – naked apart from his shoes – touching himself in the middle of the day close to a school.
The incident happened on a footpath between Quarry Hill and Malthouse Road between 12pm and 12.15pm on Tuesday June 4.
The path runs alongside Hallam Fields Junior School playing fields.
A police spokesman said: “Two 16-year-old girls were walking along the path when they saw a man standing naked apart from a pair of shoes.
“When he saw them he ran into the bushes. When they turned around they saw that he had wrapped an orange towel around himself.”
The man is described as white, aged between 17 and 18, 5ft 6ins tall, of average to chubby build and clean shaven.
He was also said to be left-handed.
Police are appealing for witnesses to call 101.