Police spokeswoman Raquel Lopez says there were people inside at the time of the 3:00 p.m. shooting, but no one was hit.
The bullet shattered a window at the West 9th Avenue office, causing an unknown amount of damage.
Lopez says investigators are looking at surveillance video but have not yet confirmed a description of a vehicle that might be linked to the shooting. Police didn't immediately release other details while detectives pursue leads.
Lopez says she isn't aware of any previous threats against the campaign office.
The Secret Service referred questions about the incident to Denver police.
An Obama campaign spokeswoman declined to comment.
(KUSA-TV © 2012 Multimedia Holdings Corporation with The Associated Press)