A witness captured the collapse on video.
The collapse occurred around 3pm on Monday, according to the UW Madison Police Department.
Authorities said between 20 and 25 people suffered minor injuries, and at least one person was taken to hospital.
The witness — a student — began filming when they noticed how crowded the pier had become.
They told ABC 27 that there was a "boom," which they initially believed to be gunshots or fireworks.
Thats when the dock began to crumble.
After the collapse, the University of Wisconsin Police Department dove around the wreck to ensure no one had been trapped under the debris.
The University of Wisconsin-Madison, one of the biggest research institutions in the United States, is investigating the incident. A spokesperson said the dock was scheduled to be removed on Tuesday to mark the end of the summer.
Elsewhere on Labor Day news, one person died and two others were hospitalised when a gunman open fired in Montgomery County, Maryland.
Police in Aspen Hill, Maryland, received a shooting report around 7.40pm. When they arrived, they found a man lying in the street suffering from a gunshot wound, Fox 5 reports.
He was transferred to hospital.
Police searched nearby residences and found two women who were also suffering from gunshot wounds. One of the women was pronounced dead at the scene, and the other was taken to a nearby hospital.
Officials said a male suspect has been taken into custody, but the investigation is still ongoing.