The $22 million project has been 17 years in the making, according to the YMCA.
It will add 23,000-square-feet of recreation, community, education and residential space, including a larger swimming pool, expanded wellness and community centers, locker rooms, a new community kitchen and a digital center for teens.
The YMCA opened in Chinatown in 1911 and will celebrate its 100th anniversary next spring. It officially reopens on Monday.
Thursday's ribbon cutting and grand opening celebration takes place at 4:30 p.m.