In the manga his gender is sometimes confused due to the Japanese language's sparse use of third person pronouns, which forces translators to use their own ideas of Deidara's gender to make sentences flow smoothly. This has caused a lot of confusion, since both "he" and "she" are used to refer to Deidara. However, Deidara speaks of "himself" using male pronouns and has been given a very deep, male voice in the anime.
Someone asked me this and it got me pretty confused. In the Naruto Wiki page it says that Deidara was a boy, but I wasn't sure.