Foxx was at the massive pop culture convention to promote his role as the villain Electro in The Amazing Spider-Man 2.
Holding his 4-year-old daughter Annalise, who wore a Spider-Man backpack and shoes, Foxx said he was "disappointed" in the 13 July not guilty verdict against George Zimmerman, who shot the unarmed black teenager in Florida.
A jury found that Zimmerman acted in self-defence.
Foxx had been among the most vocal celebrities expressing support for Martin's family, having met the teen's mother Sybrina Fulton at an awards show.
"She's always been courageous in saying this has never been about race. She said it's about 17-year-old kids. We have to protect our kids. So I stand with her forever," Foxx said.
"It was great to see Bruce Springsteen in Ireland dedicate a song to Trayvon. I think that's what really makes it universal in the fact that we know that there's race involved, but to see all races coming together and saying that hey, there's something wrong," Foxx said.
"There's something wrong when a 17-year-old child is on his way home and someone with a gun pursues him and he ends up losing his life."
Foxx said Martin's case was part of an "epidemic" of gun violence in the US.
Samuel L Jackson was also on hand at the convention, promoting the remake of RoboCop. He said he'd been out of the country for much of the trial and during the verdict, but expected the result.
"I'm not really surprised by it considering the way the case was presented and the representation that the family had, and the portrayals that they put out there of the kid and how peoples' attitudes are about those particular things," Jackson said.
Still, he said, he was "encouraged by the attitude of people after the verdict, that people are willing to stand up and take a stand and get out in the streets, and let their voices be heard."
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