Ubogagu, who scored the winner in the 89th minute to give the United States the CONCACAF title over Canada in March, sprinted on to Crystal Dunn's cross and headed the arcing ball from the six at the back post. North Korea goalkeeper O Chang Ran was able to get a hand to it, but not enough to stop from dropping inside the goal.
The victory sets up a rematch on Tuesday with Nigeria, who beat Mexico in their quarterfinal on Thursday and also eliminated the United States in the first knockout round two years ago.
The Americans outshot North Korea 14-7 although the Asian side, which included 10 members of the London Olympic team, had slightly more possession.
Vanessa DiBernardo gave the United States the lead in the 52nd, finding time and space to settle a lazy defensive clearance from Jon Myong Hwa with two touches and launch a 35-meter, right-footed strike that sailed in the left side netting past a diving O.
Kim Su Gyong equalized with 15 minutes remaining in regulation, taking a short ball slipped behind the U.S. defense at the top of the area from Kim Un Hwa, and finishing high with a left-footed shot past Bryane Heaberlin.
This will be the fifth time in six tournaments that the United States, a two-time champion, has reached the semifinals at the U-20 Women's World Cup.