The incident happened around 1:30 p.m. Saturday at Memorial Park, police say.
The victim told investigators she met the man through Whisper, a social media service. The man contacted her through the site's phone app earlier in the week, and she agreed to meet him.
According to its description in Apple's app store, Whisper allows users to "connect with likeminded individuals, and discover the unseen world around you. With Whisper, you’re free to anonymously share your thoughts with the world, and build lasting, meaningful relationships in a community built around trust and honesty."
Police describe the man's behavior in the park as "sexually aggressive." He touched her inappropriately and punched her in the face when she resisted his advances. He then left the area.
Officers say they tracked the man down using another social media site based on information he gave the victim. With help from De Pere police, authorities found the man at his home and arrested him.
The man has not yet been formally charged.