Brian D. Sicknick "was responding to the riots on Wednesday at the US Capitol and was injured while physically engaging with protesters", Xinhua news agency quoted the USCP as saying in a statement issued on Thursday night.
"He returned to his division office and collapsed," the statement added.
Sicknick's death will be investigated by the Metropolitan Police Department's Homicide Branch, USCP and "our federal partners", it added.
Earlier on Thursday, US media reports had said that a USCP officer died from Wednesday's violent riots at the Capitol building, taking the total death toll of the violence to five.
However, the USCP then denied the reports, noting that "although some officers were injured and hospitalised yesterday, no USCP officers have passed away".
Protesters in support of President Donald Trump's futile efforts to reject the 2020 presidential election results breached the Capitol on Wednesday afternoon, when Congress met to affirm President-elect Joe Biden's Electoral College victory.
Chaos and violence forced an hours-long lockdown on the Capitol grounds.