"The department prepared in order to meet these challenges, but we did not do enough," said Yogananda Pittman, who took over when the ex-chief resigned.
Despite "strong potential for violence" the force did not adequately prepare for a "terrorist attack," she said.
Five people, including a Capitol Police officer, died in the riot.
Chief Pittman was speaking on Tuesday to the House Appropriations Committee, which oversees the police department's funding.
She said the agency was aware that protesters were planning to bring firearms and other weapons to the Capitol two days before the violence on 6 January.
Her force of 1,200 officers on duty that day was "no match" for "the tens of thousands of insurrectionists" who stormed the Capitol, she said.
"The department failed to meet its own high standards as well as yours," she told the committee at the closed door meeting, according to a transcript of her remarks provided to BBC News.
"I am here to offer my sincerest apologies on behalf of the department," added Chief Pittman, who was not yet in charge of the department on the day of the attack.
Two days before the violence, former Capitol Police chief Steven Sund had requested that a state of emergency be declared, she said.
Chief Pittman said the Capitol Police Board denied that request and encouraged Mr Sund to contact the National Guard, which she said he did.