Both sides had chances in the opening 30 minutes, but it was Honduras that had the best one just before the half-hour with Josh Sargent making a great play to head the ball off the goal line following a free kick.
Zack Steffen made three saves for the U.S. None was bigger than in the 51st minute after defender Sergino Dest kept Anthony Lozano onside. Lozano had a 50-yard sprint before Steffen charged out 12 yards to make a kick save on the breakaway attempt.
The second half was also lacking fireworks and goals until substitute Siebatcheu headed a Weston McKennie assist past Lopez just before full-time to give the U.S. a lead they would hang on to for the rest of the match.
Siebatcheu was born in Washington, D.C., grew up in France and was eligible to play for both those nations and Cameroon. He made his competitive debut as a 78th minute substitute, part of a new-look American team trying to recover from the failure to qualify for the 2018 World Cup. He is now tied to the U.S. team.
"It was a little bit nerve-wracking," Gio Reyna, the 18-year-old son of former American captain Claudio Reyna, said after making his competitive debut. "It was really important. It was the first, tough really official competitive game we've had."
The 11 starters for the match averaged 23 years, 259 days for the second youngest lineup for the U.S. in an official competition, bettered only by the 23 years, 172 days for a match against Colombia on July 5, 2007.
The U.S. will face Mexico in Sunday's Nations League final after El Tri beat Costa Rica in the other semi. It's the last preparation for regular national team players on all squads ahead of the start of World Cup qualifying in September.