A jury in Wilmington, Delaware, federal court found him guilty on all three counts against him, making Hunter the first child of a sitting US president to be convicted of a crime.
A verdict of the 12-member jury must be unanimous on each count.
The trial followed the May 30 criminal conviction of Trump, the first former US president to be found guilty of a felony and the Republican challenger to Joe Biden, a Democrat, in the Nov 5 election.
Trump, convicted on 34 felony counts of falsifying business records to cover up a sex scandal, accuses Democrats of pursuing that case and three other criminal prosecutions to prevent him from regaining power in his rematch with Joe Biden.
Congressional Democrats have pointed to cases including Hunter’s prosecution as evidence that Joe Biden is not using the justice system for political or personal ends, having said last week he would not pardon his son if convicted.
The Hunter case was brought by US department of justice special counsel David Weiss, a Trump appointee.
Weiss has also charged Hunter with three felony and six misdemeanour tax offences in California, alleging he failed to pay US$1.4 million in taxes between 2016 and 2019 while spending millions on drugs, escorts, exotic cars, and other high-ticket items.
Hunter has pleaded not guilty to those charges. A trial is scheduled for Sept 5 in Los Angeles.
The Delaware trial included prosecution testimony by Hunter’s ex-wife, former girlfriend, and sister-in-law, who gave first-hand accounts of his spiralling addiction in the weeks before and after he bought the gun in October 2018.
Prosecutors also showed text messages, photos, and bank records that they said showed Hunter was deep in the throes of addiction when he bought the gun and knowingly broke the law by answering “no” to being a drug user on a government screening form.
Hunter’s lawyers sought to show that he was not using drugs when he bought the gun and did not intend to deceive because he did not consider himself a drug user when he filled out the form.
The defence called Hunter’s daughter, Naomi, who testified that her father seemed to be doing well when she saw him shortly before and after he bought the gun.
The sentencing guidelines for the gun-related charges against Hunter are 15 to 21 months, but legal experts say defendants in similar cases often get shorter sentences and are less likely to be incarcerated if they abide by the terms of their pre-trial release.