Aidan P. O'Brien (born 16 October 1969 in County Wexford, Ireland) is an Irish horse racing trainer. Since 1996, he has been the private trainer at Ballydoyle Stables near Cashel in County Tipperary for John Magnier and his Coolmore Stud associates.

Aidan O'Brien was one of six children of Denis O'Brien (d. 1 December 2008) and his wife Stella (née Doyle). Denis was a farmer and small-scale horse trainer in the townland of Killegney, near Poulpeasty, in Co. Wexford, where Aidan grew up.

Aidan O'Brien attended Donard National School, located less than a mile from his parents' home. He subsequently attended secondary school at Good Counsel College, New Ross, also located in Co. Wexford.