The We Need to Talk About Kevin star had previously never mentioned his sexuality in interviews, yet broached the subject while promoting his new film The Perks of Being a Wallflower, where he plays a gay character alongside Emma Watson.
Miller told Out Magazine: "I'm queer. I have a lot of really wonderful friends who are of very different sexes and genders. I am very much in love with no-one in particular."
The actor then went on to detail some of his philosophies regarding relationships, saying: "I don't know if it's responsible for kids of my age to be so aggressively pursuing monogamous binds, because I don't think we're ready for them.
"The romanticism within our culture dictates that that's what you're supposed to be looking for. Then [when] we find what we think is love - even if it is love - we do not yet have the tools. I do feel that it's possible to be at this age unintentionally hurtful, just by being irresponsible - which is fine.
"I'm super down with being irresponsible. I'm just trying to make sure my lack of responsibility no longer hurts people. That's where I'm at in the boyfriend/girlfriend/zefriend type of question."
Watch the trailer for The Perks of Being a Wallflower: