Fashion's biggest night was the first public outing the low-key couple has made together, though they didn't pose together on the red carpet.
Holmes, 40, and Oscar winner Foxx, 51, have been linked since 2013, but have never confirmed their romance publicly.
The erstwhile "Dawson's Creek" actress wore a custom floor-length, mermaid-style purple Zac Posen gown with a magenta collar of 3D printed palm leaves. Foxx matched his purple dress shoes perfectly to his paramour's violet dress.
A source told People of the pair, “When they can spend time together, they do. When they’re busy and they can’t, they don’t. They are two adults who enjoy each other’s company and have for a long time.”
“Katie and Jamie see each other as frequently as they can,” the source insider said. “They have known each other for years, and both turned out to be great parents, so they also have that in common. They are dedicated to their careers. Plus they give each other space.”