“Swelling of the penis” is a very common, although undefined, term. Many conditions such as tumors, infections, allergic reactions, injuries etc. can cause swelling. The swelling that you are experiencing can be followed by presence or lack of pain, redness, warmness, disturbed function (erection and peeing), urethral discharge, etc. Infections and allergic reactions are treated with medications. Tumors are treated surgically. You can visit a urologist for a thorough examination of your penis.