The actor, 31, proposed to Loni Willison during a romantic dinner in Los Angeles on Halloween (31Oct11).
He tells America's Star magazine, "Loni's always hated Halloween, so I thought I'd change her perspective on it for the rest of her life.
"We had a private room with candles everywhere. I had a violinist play Have You Ever Really Loved a Woman? She teared up and said yes. I was the most amazing day of my life."
Jackson's proposal took place just months after he kicked his addiction to steroids following a stint on U.S. reality show Celebrity Rehab earlier this year (11).