Billed as the largest student union in the world,[1] the Oklahoma State University Student Union is a student activity center for meetings, conferences, meals, recreation, and shopping for students and alumni on the campus of Oklahoma State University in Stillwater, Oklahoma.

Five major expansions of the Union took place in 1960, 1965, 2000, 2012 and 2016 with an adjacent parking garage built in 1970.

The Student Union was closed during the 2010 - 2011 academic school year for renovation; with the majority of student services relocated to several places around campus like the Classroom Building basement or the "UToo" temporary building.

The Atherton is named for Oklahoma State alum Bill Atherton and serves as a living classroom for Hotel and Restaurant Administration students while providing luxury boutique hotel services for guests.

The Union is also home to the luxurious Ranchers Club restaurant, a 550-seat theater, art exhibits, lounges, meeting rooms, an extensive food court, bookstores, the OSU Student Store, a recreation center, post office, travel agency, athletic ticket office, and multiple university offices.