The blaze began a little before 7:30 a.m. on the third floor of the Capital Square building, which counts The Des Moines Register newspaper among its tenants.
Flames and some shattered windows could be seen from the street.
Fire Department spokesman Rick Thomas says most of the flames had been knocked down by a little after 8 a.m. He told the newspaper that fire sprinklers checked the blaze until firefighters could begin their work.
The fire cause is being investigated.
