"It is an incredible honor to vote for my father! He will do such a great job for the U.S.A! #MakeAmericaGreatAgain," Eric Trump wrote Tuesday morning in the tweet, with an apparent image of his ballot, which was later taken down.
New York state law makes it a misdemeanor for one person to show their completed ballot to another person -- although the law, which dates to the late 1800s, does not account for Twitter or any other social media.
The Trump campaign did not immediately respond to a request for comment.
"Although it's more than a little ironic that someone as supposedly well-informed as Eric Trump doesn't know what the laws are when it comes to ballot selfies, it's also an important reminder of just how antiquated these laws are — and how much they vary from one state to the next," said Steve Vladeck, a CNN contributor and professor of law at the University of Texas School of Law.