The 17-year-old kid actor will wed Rachel Miner--also a kid actor, also 17, the couple's publicist says.
No date has been announced.
Culkin and Miner, currently a costar in the Broadway production of The Diary of Anne Frank, say, in a statement, that they're "happy and proud" to have found each other.
The two began dating in 1997.
The role of groom will be Culkin's biggest in years. His last film was the 1994 bomb, The Pagemaster.
After a four-year run as Hollywood's most powerful child star since Shirley Temple, Culkin's career was torpedoed by family troubles. (Temple, incidentally, also married for the first time at age 17.)
In 1995, his father/manager, Kit, and mother, Patricia, split (even though they'd never married) and began a bitter war over Macaualay's millions and custody issues. Soon after, Culkin, who once commanded $5 million for Home Alone 2: Lost in New York, was more likely to be sighted in a New York nightclub than a movie set.
Kit Culkin, now his kid's ex-manager, was notorious for strong-arm negotiations, even by Hollywood standards. In one infamous incident, he refused to commit Macaulay to the first Home Alone sequel unless his child was cast as the evil kid in the 1993 film, The Good Son.
Last year, a judge declared Kit Culkin a "demanding, difficult and pugnacious man" and awarded custody of Macaulay and three siblings to their mother.
Macaulay Culkin legally becomes his own man August 26, when he turns 18.
Culkin recently flexed his acting muscles in public again, participating in a staged play reading in New York.
Miner is a veteran child of show biz in her own right. She played Michelle Bauer on the CBS soap, The Guiding Light, from 1989-1995.