The Bible does not address this issue directly, but Jesus is clear on the issue of lust and adultery. He says that if you look at a woman lustfully, then you have already committed the sin of adultery with her in your heart (Matthew 5:28). The point is that your inward motivations and thoughts are just as sinful as your actions.

While we cannot say that masturbation is a sin per se, we can say that lust, which normally is associated with masturbation, is a sin. The Bible is clear that sexual activity is for married people. Sex outside the covenant of marriage is sin. Search your heart honestly and prayerfully, and if you feel convicted about masturbation, then find a mature person in the church who you can talk to and who can hold you accountable.

For more information check out Chapter 5 from Mark Driscoll’s E-Book: Porn Again Christian. http://relit.org/porn_again_christian/ch5.php