The two split earlier this year. Grande is now engaged to Pete Davidson, a cast member on "Saturday Night Live."
"I really can’t wrap my head around it. We talked about this. So many times. I’m so mad, I’m so sad I don’t know what to do. You were my dearest friend. For so long. Above anything else." the singer wrote.
Grande also shared a video she recorded of Miller and her.
"I’m so sorry I couldn’t fix or take your pain away. I really wanted to."
TMZ was first to report Miller's death. In a recent report, TMZ, citing law enforcement sources, said Miller may have been dead for hours before his body was discovered at his California home.
A cause of death has not yet been determined, TMZ reports.