Koutoujian, a Waltham Democrat appointed sheriff in 2011 after 14 years as a state representative, would enter a thickening field that is banking on a Markey victory opening the US House seat.

“This is the right decision for me,” Koutoujian said. “This is something that I’ve looked at in the past. It is something about which I feel very strongly, and I think I can bring over 20 years’ experience as a prosecutor, state lawmaker, and sheriff.”

A former prosecutor, Koutoujian won the sheriff’s job outright in November. On Thursday, a public relations firm said Koutoujian had opened a federal campaign committee account, allowing him to raise money and hire campaign staff “when appropriate.”

“I believe my stances on the issues are in line with what the people of the Fifth Congressional District are looking for,” Koutoujian said.

If Markey beats Gomez, a special election to fill his US House seat would take place in the fall, state officials said. The potential field also includes state Senators William Brownsberger of Belmont, Katherine Clark of Melrose, and Karen Spilka of Ashland and state Representative Carl Sciortino of Medford, all Democrats.