Hickenlooper announced his run by video message on Thursday. He becomes the front-runner in a crowded Democratic primary field competing for the right to face Gardner, considered the most vulnerable Republican senator in the country.
Hickenlooper initially brushed off Democratic entreaties to run against Gardner. Instead he mounted a longshot presidential campaign that collapsed before it ended in mid-August.
The Democrats already in the Senate race are younger, and they have indicated they won't go quietly. One said the primary "won't be a coronation."
Hickenlooper remains widely popular and well known with Colorado Democrats, but Republicans hope Democratic Party's generational struggles will wound him in the primary.