Four others were injured in the workplace shooting, Hennepin County Sheriff Rich Stanek told KMSP-TV, Minneapolis.
The shooting was reported to police at 4:45 p.m. at Accent Signage Systems Inc. on Chestnut Avenue West, in the Bryn Mawr neighborhood, the TV station said.
Police say the shooter was known to them before Thursday's incident, and they have information he may have targeted specific employees.
Police found the gunman dead at 5:30 p.m. after a 30-minute sweep of the building by SWAT and Minneapolis police.
"Very sad situation in Bryn Mawr," Minneapolis Mayor R.T. Rybak posted on Twitter. "Please stay away and let the police do their work."