Morgan Brian scored in the first half and Kealia Ohai added another after the break as the U.S. advanced to the final for the third time, having won the tournament in 2002 and 2008.
"It was a difficult match physically," U.S. coach Steve Swanson said. "Nigeria gave us all we could handle and a little more. Getting that first goal was key for us."
Brian opened the scoring with a header in the 22nd minute and Ohai doubled the lead in the 70th, taking a through ball from Samantha Mewis and beating Nigeria's goalkeeper Ibubeleye Whyte with a right-foot shot from 12 metres (yards).
Nigeria pressed forward after the second goal but the Americans denied them a chance to get back in the game.
"I thought our back four did a very good job of keeping possession and allowing us to get out of our end," Swanson said.
Germany, looking to become the first team to win consecutive titles, scored three goals in the opening 20 minutes in beating host Japan 3-0.
Melanie Leupolz opened the scoring in the first minute when she took a pass from Dzsenifer Marozsan and beat Japan goalkeeper Sakiko Ikeda with an angled right-foot shot.
Marozsan doubled the lead in the 13th with a lob over the head of Ikeda and Lena Lotzen made it 3-0 with a header in the 19th for her sixth goal of the tournament.
Japan will face Nigeria for third place on Saturday before the U.S. meets Germany in the final. Germany also won the title in 2004.