A third party reported seeing a man touch the victim while she was walking in the Market Street area of Loughborough around 2am.
From enquiries conducted so far it’s believed she has not reported the incident to the police.
Officers are urging the victim to get in touch so they can continue their enquiries.
A 25-year-old man from the Birmingham area has been arrested on suspicion of sexual assault and released on bail pending further enquiries.
Anyone who can assist with the investigation is asked to contact DC 1892 Del Gillespie on 101 or via www.leics.police.uk, quoting crime number 22*356520.