Police said a car crashed into a security barrier before the driver lunged towards the officers with a knife. The officers opened fire and the suspect was shot dead.
The attack took place shortly after 1pm local time (6pm GMT), when a man driving a blue sedan rammed the car into two officers standing at the North Barricade, according to police.
However, authorities said the attack did not appear to be terrorism-related.
‘We have a responsibility to get to the bottom of it and we'll do that’
At a news conference, Robert Contee, the acting chief of Washington DC's Metropolitan Police Department, said: "Whether the attack was at law enforcement, or whoever, we have a responsibility to get to the bottom of it and we'll do that.”
Police said the suspect died due to his injuries at 1.30pm local time, but Chief Contee told reporters the suspect appeared to act alone.
The FBI's Washington Field Office also said it was providing support to the Capitol Police.
Acting Capitol Police Chief Yogananda Pittman said: “The suspect did start lunging towards US Capitol police officers, at which time US Capitol police officers fired upon the suspect.
"At this time the suspect has been pronounced deceased.
"Two US Capitol police officers were transported to two different hospitals and it is with a very heavy heart that I announce one of our officers has succumbed to his injuries."
In a later statement, Chief Pittman named the officer as William "Billy" Evans, who was part of the department's first responders unit and had been a member of the Capitol Police for 18 years.
She said: "Please keep Officer Evans and his family in your thoughts and prayers," she said.
This attack comes three months after January's Capitol riots, which saw a wave of Donald Trump supporters storm the US Capitol building as Congress met to certify November’s presidential win for Democrat Joe Biden.