A hamstring (/ˈhæmstrɪŋ/) is any one of the three posterior thigh muscles in human anatomy between the hip and the knee: from medial to lateral, the semimembranosus, semitendinosus and biceps femoris.

The word "ham" is derived from the Old English “ham” or “hom” meaning the hollow or bend of the knee, from a Germanic base where it meant "crooked". It gained the meaning of the leg of an animal around the 15th century. String refers to tendons, and thus the hamstrings' string-like tendons felt on either side of the back of the knee.

The common criteria of any hamstring muscles are:

Those muscles which fulfill all of the four criteria are called true hamstrings.
The adductor magnus reaches only up to the adductor tubercle of the femur, but it is included amongst the hamstrings because the tibial collateral ligament of the knee joint morphologically is the degenerated tendon of this muscle. The ligament is attached to the medial epicondyle, two millimeters from the adductor tubercle.

The three muscles of the posterior thigh (semitendinosus, semimembranosus, biceps femoris) flex (bend) the knee, while all but the biceps femoris extend (straighten) the hip. The three 'true' hamstrings cross both the hip and the knee joint and are therefore involved in knee flexion and hip extension. The short head of the biceps femoris crosses only one joint (knee) and is therefore not involved in hip extension. With its divergent origin and innervation, it is sometimes excluded from the 'hamstring' characterization.

A portion of the adductor magnus is sometimes considered a part of the hamstrings.

The hamstrings cross and act upon two joints – the hip and the knee – and as such they are termed biarticular muscles. The hamstrings contract when the knee is bent, and lengthen when the knee is extended, and when the hips are extended.

Semitendinosus and semimembranosus extend the hip when the trunk is fixed; they also flex the knee and medially (inwardly) rotate the lower leg when the knee is bent.