She made the commitment after her campaign was backed by America's largest LGBT rights organisation.
Gay conversion therapy is help from a psychotherapist for a person struggling with unwanted same-sex attraction.
It is supported by a number of church groups both in the US and UK.
In a Tweet, Mrs Clinton said: "LGBT kids don't need to be "cured," they need to be loved and supported."
LGBT kids don't need to be "cured," they need to be loved and supported. pic.twitter.com/GixB3AJrUV
— Hillary Clinton (@HillaryClinton) January 19, 2016Clinton has made LGBTI rights a key part of her election campaign.
She said: "I've been fighting alongside you and others for equal rights and I'm just getting warmed up, That's a promise, from one HRC [Human rights campaigner] to another."
The Education Secretary Nicky Morgan has expressed her desire to ban gay conversion therapy in the UK.