US Capitol Police officer Brian Sicknick died of injuries suffered when supporters of President Donald Trump assaulted the legislative building, bringing to five the number dead from the riot.
Wednesday's breach of the US Capitol building was a stunning assault on American democracy when lawmakers were in the building certifying the victory of President-elect Joe Biden.
"Officer Sicknick was responding to the riots...and was injured while physically engaging with protesters," police said in a late-night statement.
He succumbed on Thursday after being taken to hospital following his collapse upon returning to his divisional office, they added.
Metropolitan homicide officials will investigate the death of Sicknick, who joined the US Capitol Police in 2008, along with the Capitol force and its federal partners, police said.
Trump, who initially praised his supporters, later condemned the violence, saying the rioters had defiled the seat of American democracy and must be held accountable.
A woman demonstrator was fatally shot by authorities, and three people died from medical emergencies.
After a rally where Trump exhorted his supporters to "fight" to overturn the November 3 election he lost, hundreds of them stormed the Capitol on Wednesday, forcing senators and members of the House of Representatives to evacuate as they smashed windows and looted.
US Transportation Secretary Elaine Chao and Education Secretary Betsy DeVos resigned on Thursday, joining a growing list of aides leaving Trump's administration in protest at the storming of the US Capitol.
As Democratic leaders demanded his ouster, Trump came closer to a formal concession, acknowledging in a video released on Thursday evening that a "new administration" would be sworn in on January 20 and vowing to ensure a "smooth transition".