Bold, salty Japanese soybean paste aged 1-3 years.
Substitutes
Doenjang (Korean Soybean Paste) — 1:1 (doenjang is slightly more pungent, reduce by 5-10%). Korean doenjang shares red miso paste's (aka miso) deep, robust fermented soybean character. Per 'The Oxford Companion to Food', both undergo long fermentation producing high concentrations of glutamic acid. Red miso (aka miso) uses a rice-soybean blend while doenjang is pure soybean, making doenjang slightly more intense and pungent. Both are used in soups, stews, marinades, and dressings. An excellent cross-cultural match for most recipes calling for red miso.
Miso — 1:1. Generic miso is typically lighter than red miso. Add a splash of soy sauce for extra depth if needed.
Hatcho Miso — 1:1 (use slightly less — hatcho is stronger). Hatcho miso is even darker and more intense. Use about 3/4 amount to avoid overpowering the dish.