Summer penis and summer penis syndrome are two different conditions.

Summer penis is not a known medical condition. It is based on the saying that during the summer the penis seems to be bigger, while during the winter smaller.

Although there is no medical support for this claim, there are a number of explanations for the claim, including:

- During the summer, people get more hydrated. Proper hydration can give the penis the look of a larger size.
- Blood vessels can dilate to regulate heat and shrink in response to cold.

Summer penis syndrome is a seasonal medical condition characterized by redness, swelling and itching of the skin of the penis. This syndrome is usually caused during the summer months by the bites of a spider mite. This condition is usually treated with oral medications, cold compresses, topical creams, etc.