The Florida rapper, a streaming juggernaut, won for his 2017 debut "17" and was also nominated for new artist of the year. Cleopatra Bernard, XXXTentacion's mother, accepted the award in his honor.
"I am honored to accept this award on behalf of my son. I would like to thank the AMAs, his fans and to everyone who made this possible: thank you guys, so much," Bernard said. "I appreciate this."
XXXTentacion, born Jahseh Dwayne Onfroy, was killed June 18, 2018, in a robbery at a motorcycle dealership in Deerfield Beach, Florida. He was 20. Four suspects have been indicted in connection with the rapper's murder.
XXXTentacion also won best new artist at the BET Hip Hop Awards earlier in October.
The Associated Press contributed to this report.