In a video from the event, Mr Trump stops singing just before the penultimate line of “The Star Spangled Banner”, despite those around him continuing to recite the song.
The president then looks around, before resuming singing for the anthem’s final line.
The incident is not the first time Mr Trump has appeared to forget the song’s lyrics.
In January 2018, critics accused him of not knowing the words to the anthem after a video showed him apparently struggling through the middle section of the song at an American football game in Atlanta.
Mr Trump has previously criticised Americans who do not sing the national anthem or choose to kneel during it and has suggested those who do not want to stand for the anthem “should not be in the country”.
At Saturday's football game in Tuscaloosa, Alabama, the president was met with a positive reception after he was loudly booed at a baseball match in Washington last month and received a mixture of boos and cheers at a UFC match in New York last week.
In the pro-Republican state of Alabama, which Mr Trump won with 62 per cent of the vote in 2016, he was greeted with loud cheers from fans.
However, there was conflict outside the stadium when a “Baby Trump” protest balloon was knifed and deflated by an apparent Trump supporter.
The baby Trump balloon was first used to protest the president's visit to London in July 2018 and replicas of the balloon have since been used across the US.
Tuscaloosa Police Department have arrested a man named Hoyt Deau Hutchinson for criminal mischief over the incident.
Robert Kennedy, a volunteer who brought the balloon to Tuscaloosa, said he had never witnessed an attack on Baby Trump before.
“It is rare to get that kind of anger,” Mr Kennedy said.