Is quinoa a grain or a seed?

Quinoa is botanically a seed. It is called a pseudocereal because people cook and use it much like a cereal grain.

Quinoa panicles and tiny harvested quinoa seeds — Quinoa is a seed crop that fills many of the same culinary roles as cereal grains.

The short answer

Quinoa comes from Chenopodium quinoa, a broadleaf plant rather than a cereal grass. Its small seeds can be simmered, flaked, puffed or milled, which gives quinoa a grain-like culinary role. “Pseudocereal” preserves both facts: it is a seed by botany and a grain-style food in the kitchen.

Sources

The Global Expansion of Quinoa: Trends and LimitsFrontiers in Plant Science / PMC