In human anatomy, the thymus is a ductless gland located in the upper anterior portion of the chest cavity. It is most active during puberty, after which it shrinks in size and activity in most individuals and is replaced with fat. The thymus plays an important role in the development of the immune system in early life, and its cells form a part of the body's normal immune system.

Children with an enlarged thymus suffer from sudden attacks of unconsciousness, convulsions, and sometimes sudden death; this condition is known as status thymicolymphaticus. Tumors of the thymus are found in about 10% of patients with myasthenia gravis.

The thymus is also present in many other animals. When animal thymus tissue is sold in a butcher shop or at a meat counter, thymus is known as sweetbread.

Thymus (family Lamiaceae) is the genus of plants to which thyme belongs.