"It is imperative that we seek the truth for what happened," Pelosi told reporters in the Capitol.
"The timetable will be as long as it takes," she said, leaving open the possibility that the probe could extend into 2022, a mid-election year, the Xinhua news agency reported.
Pelosi did not reveal details about the composition of the committee, or who she'll choose to lead it.
Senate Republicans last month blocked legislation to establish an independent panel modelled on the 9/11 commission to conduct the probe.
Five people, including a Capitol police officer, died in the riot when a mob of former US President Donald Trump's supporters breached the Capitol to stop Congress from certifying President Joe Biden's election victory. More than 400 people have been charged with crimes related to the Capitol attack.