Vlatko Andonovski's side was frustrated by a well-organized Portugal team that almost snatched victory in stoppage time when striker Ana Capeta hit the post -- a strike that, had it gone in, would have sent the USWNT crashing out of the tournament.
The USWNT will face the winner of Group G in Melbourne on Sunday in its first knockout match, likely to be Sweden, which needs just a point against bottom-placed Argentina to confirm top place and the matchup.
"We should have put some in the back of the net and we didn't. We own that," Alex Morgan said. "We're not happy with the performance we put out there. But at the same time, we're moving on."
Defender Lynn Williams had a headed chance after 13 minutes but watched on as Portugal goalkeeper Ines Pereira smothered the attempt. While the U.S. controlled possession in the first half and had the better chances, it could not find the right finish, and the game was scoreless at the break.
Rose Lavelle picked up a yellow card in the 38th minute, her second of the group stage. She will not be available for the round-of-16 clash.
Early in the second half, a malfunctioning sprinkler caused a fire alarm to go off in the stadium, but play went on as the U.S. continued to be frustrated.
Megan Rapinoe came on as a substitute after 60 minutes but could not make an impact on the score line.
"Obviously we want to play great and score a lot of goals, and we didn't do that. We know that it can be better, and has to be better, moving forward," Rapinoe said "But ultimately, we're on to the next round."
Capeta's effort in stoppage time capped a spirited late attack from Portugal, which exits the competition.
Portugal manager Francisco Neto said: "I truly believed that the goal would be in that moment and I was starting to think, `What can I do to help my players if we're winning 1-0?' What I said to the girls, I was very proud. Of course, they are very sad because we have huge expectations of ourselves."
The result means the U.S. finishes in second place in Group E on five points -- two behind Netherlands -- but will have the opportunity continue its bid for an unprecedented third consecutive Women's World Cup title.
Information from The Associated Press contributed to this report.
- Rebecca Welch