There is some debate about that. I can recommend books for the "yes" and "no" and the "it's complicated" perspectives.
Sex Addiction is not in the DSM-V, the Diagnostic and Statistical Manual of the American Psychiatric Association, and some authors say that it is not an addiction and that we should be careful labelling behaviours such as sex or masturbation with porn as such. Some people say that it is an addiction and there are therapists and support groups that seek to help people regain control of their lives.
An addiction is a chronic disorder in which an individual is blinded by a substance, or engages in an activity that gives them an immense amount of pleasure but has become detrimental to their everyday life. Compulsive behaviours and addictions can temporarily provide confidence, control, relief, validation or other emotions lacking in one’s life, but the behaviour may not stop until the root of the problem is addressed.
Addiction changes the brain through the limbic system, also known as the “brain reward system.” This part of the brain is responsible for producing feelings of pleasure and will manifest thoughts such as, “I deserve this,” “let’s do that again", or "just this time", or as with video game addiction, "one more turn". The abuse of addictive substances and behaviours triggers this system which can prolong a continuous cycle of destructive behaviour.
So, if sex and/or porn have become problematic for you, and your behaviour is something that you want to, or feel that you should, change, then it can be helpful to approach it like an addiction AND to explore what led that behaviour to develop in the first place. One key difference as compared with other addictions however, is that while many addiction and recovery programs, including groups like Alcoholics Anonymous, promote abstinence as recovery, that is not the case with sex. A healthy sexuality that you are comfortable with is usually the goal, and for most guys that involves sex and masturbation. With pornography however, abstinence may be useful, at least for a while.
An addiction of any sort can be exhausting and one should never go through the recovery process alone.
There are a number of addiction recovery groups which people may find helpful as a complement to therapy. Check out the Links & Resources page of this site, or ask about joining a small (5-6 person) discussion group on the topic of your choice.