Harry Dunn, a former U.S. Capitol Police officer who rose to prominence for his defense of the Capitol on Jan. 6, 2021, and for his emotional public testimony describing the attack, announced on Friday that he was running for Congress in Maryland’s third district.

“On Jan. 6, 2021, I did my duty as a police officer and as an American and defended our nation’s Capitol from violent insurrectionists,” Mr. Dunn said in a statement. “Today, I’m running for Congress because the forces that spurred that violent attack are still at work, and as a patriotic American, it is my duty to defend our democracy.”

Mr. Dunn, 40, will enter a crowded Democratic primary field to replace Representative John Sarbanes, the retiring 17-year incumbent. Five state lawmakers have already announced their campaigns to represent the central Maryland district, which snakes between Washington and Baltimore. Whoever emerges from the primary in the overwhelmingly Democratic district is almost certain to win the general election.

Mr. Dunn, a member of the Capitol Police for 15 years, was one of four officers who testified at the first public hearing of the House committee that investigated the attack by the pro-Trump mob on the Capitol, where lawmakers had gathered to certify the results of the 2020 presidential election. He described how fellow officers bloodied in the battle and how rioters used racist slurs against him.

“I sat down on a bench with a friend of mine who is also a Black Capitol Police officer and told him about the racial slurs I had endured,” Mr. Dunn recalled during a memorable portion of the testimony. He added that he “became very emotional,” asking how such a thing could happen and yelling, “‘Is this America?’”

“I began sobbing, and officers came over to console me,” he said.

In 2023, President Biden awarded Mr. Dunn the Presidential Citizens Medal in recognition for his role in protecting the Capitol.

Mr. Dunn grew up in the Washington suburbs of Prince George’s County, Md., and graduated from James Madison University in Virginia, where he played football and helped lead the team to its first national title.

He has written a book called “Standing My Ground.”