Bush was to attend the event Saturday. His spokesman, David Sherzer, said Friday that Bush doesn't want to share a forum with someone who has "willfully and repeatedly done great harm to the interests of the United States."
Assange is in Britain, fighting extradition to Sweden in a sex crimes inquiry. He is free on bail, but it wasn't immediately clear if he ever planned to be in Denver.
WikiLeaks has release of tens of thousands of secret U.S. government and military documents.
Sherzer says Bush learned of the invitation to Assange this week.