Police are appealing for witnesses to a sex attack on a female jogger in Ferntree Gully earlier this month.

The woman, 22, told police she was jogging on Scoresby Road about 6.30pm on April 14 when a man ran up behind her and grabbed her buttocks with one hand and her inner thigh with his other hand.

Police said the woman screamed, broke away from the man and continued along Scoresby Road.

She told investigators that she had noticed the man in the lead-up to the attack walking along Scoresby Road near Sandgate Boulevard.

The man is believed to be thin, Caucasian, 170cm tall and aged in his 40s.

He was wearing cream chino pants, a black knitted top with a collar and round-framed glasses.

Anyone with information should phone Crime Stoppers on 1800 333 000 or visit www.crimestoppers.com.au.