I have a small bb size knot (actually feels like a small bb) under my skin on my penis, what could it be?

Lots: You should see your dermatologist for treatment and evaluation because this way you can get the correct diagnosis and treatment.

Cyst: It may depend on the location but underneath asking sounded like and plugged gland or a sebaceous cyst. But if you have an exposure to sex (occasional) then it may be something more like a venereal infection but would have had more symptoms bottom line you need to get it check for an urologist if is a new growth.

Cyst or other: Hello mikek, a small bb size knot is generally a benign cystic lesion as a result of trauma, blockage of ducts, or inflammation. Rarely, it can be a malignant lesion. I would advise having a physician lay his or her hands on the skin of the penis itself, sooner rather than later. Hope that helps.