A mature jalapeño chile is 5–10 cm (2–4 in) long and hangs down with a round, firm, smooth flesh of 25–38 mm (1–1+1⁄2 in) wide. It can have a range of pungency, with Scoville heat units of 3,500 to 8,000. Commonly picked and consumed while still green, it is occasionally allowed to fully ripen and turn red, orange, or yellow. It is wider and generally milder than the similar Serrano pepper.

A raw jalapeño is 92% water, 6% carbohydrates, 1% protein, and contains negligible fat. A 100-gram (3+1⁄2 oz) reference serving of raw jalapeños provides 120 kilojoules (29 kcal) of food energy, and is a rich source (20% or more of the Daily Value, DV) of vitamin C, vitamin B6, and vitamin E, with vitamin K in a moderate amount. Other micronutrients are low in content