Mr Trump's supporters said they were proud to see him standing with his hand over his heart, while his critics said it looked as though he had forgotten the lyrics.
Did he forget the words?
He certainly didn't sing all of them.
Video of the anthem shows Mr Trump getting off to a slow start. He appears to mouth some of the words while occasionally pausing and missing some out.
Some lines and phrases are sung with gusto. The president can clearly be seen mouthing "bright stars" and he finishes with a flourish, smiling and singing "the land of the free and the home of the brave".
Other lines are not sung at all. Whether or not he forgot the words is open for debate. He has been pictured singing the anthem in the past and has been vocal in his support of respecting it.
The BBC's Anthony Zurcher has a theory that singing along in this instance is difficult for two reasons:
For those grousing about Trump seemingly not knowing the words to the national anthem: 1) He's hard of hearing and 2) down on the field, the sound bounces around a lot and it's hard to decipher the music. Singing along wouldn't be easy for him.
What is the US president expected to do?
The Flag Code covers aspects of etiquette when the anthem is played in some detail. The code is never enforced, however, and there is no punishment for breaching it.
In the video, the president can be seen standing in the customary way, as stipulated by the code. It states that:
During a rendition of the national anthem when the flag is displayed all present except those in uniform should stand at attention facing the flag with the right hand over the heart.
By contrast, his predecessor President Barack Obama famously neglected to put his hand over his heart during the 2008 election campaign. He said his grandfather had taught him to do this only during the pledge, and to only sing during the anthem.
The code adds that military personnel and veterans should salute throughout the anthem.
It makes no mention of whether the anthem should be sung. However, singing the Star-Spangled Banner is encouraged from school onwards - in some schools it is compulsory. In 1942 the National Anthem Committee wrote that "it is of paramount importance that emphasis be placed upon the singing of the National Anthem".
Ultimately, it comes down to personal choice as to whether the president sings the anthem or just stands respectfully.
Barack Obama has done both, and said in 2016 that "respecting and honouring our flag and our anthem is part of what binds us together as a nation".
Former President Ronald Reagan once said: "I can't claim to know the words of all the national anthems in the world, but I don't know of any other that ends with a question and a challenge as ours does."
What has Mr Trump said about the anthem?
The president has been extremely vocal in urging respect for the US national anthem.
On Monday, he tweeted: We want our flag respected - and we want our NATIONAL ANTHEM respected also!". He has even changed the banner image on his Twitter account to a photograph of him standing during the anthem before the game.
Mr Trump has also strongly criticised NFL stars kneeling during the anthem in protest against perceived racial injustice. He has previously insisted that players who fail to stand should be fired or suspended.
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