For the first time, a female sailor has completed the US Navy training programme to become a Special Warfare Combatant-craft Crewman (SWCC).
Members of the elite US defence force group support Navy SEALs in high-risk warfare missions, and conduct their own classified military operations.
The sailor who completed the gruelling 37 weeks of training has not been named, in line with Pentagon policy.
The US military began allowing women to serve in combat roles in 2015.
The troop was among 17 sailors to graduate the "assessment and selection" process on Thursday, the Navy said in a news release.
Only about 35% of sailors who apply to the SWCC programme manage to complete it, officials say.
The gruelling exercises train recruits on weaponry, navigation, parachuting, combat and covert insertion and extraction - getting soldiers in and out of hostile or classified areas.
The programme culminates in a 72-hour event called the Tour, which is the point when many troops drop out. It tests recruits both physically and mentally as they endure 23 hours of running and 5 miles (8km) of swimming in challenging environments.
"Becoming the first woman to graduate from a Naval Special Warfare training pipeline is an extraordinary accomplishment, and we are incredibly proud of our teammate," said Rear Admiral HW Howard, Commander of US Naval Special Warfare Command.
"Like her fellow operators, she demonstrated the character, cognitive and leadership attributes required to join our force."
According to the Associated Press, the troop is the first of 18 women who have applied to be a SWCC or a SEAL to succeed.
Of the those, 14 were unable to complete the course, and three are currently still undergoing training.