A bunion, also known as hallux valgus, is an outward deformity of the foot's metatarsophalangeal (MTP) joint which connects the big toe to the foot. The rear tarsametatarsal joint that holds the metatarsal bone in a straight-ahead position weakens, the metatarsal moves outward plus rotates 90 degrees bringing the sesamoids up against the adjacent toe. This results in the head of the metatarsal bulging outward, and the big toe then bends inward toward the other toes. The joint often becomes red and painful due to rubbing in a cramped shoe. The onset of bunions is typically gradual. Joint complications may include bursitis or arthritis. A similar condition of the little toe is referred to as a bunionette.

Treatment may include proper shoes, orthotics, or NSAIDs. If this is not effective for improving symptoms, surgery may be performed. Since 2020, many new and highly effective surgical techniques for bunion correction have been developed. Bunions exist in about 23% of adults. Females are affected more often than males. Usual age of onset is between 20 and 50 years old. The condition also becomes more common with age. It was first clearly described in 1870. Archaeologists have identified a high incidence of bunions in skeletons from 14th- and 15th-century England, coinciding with a fashion for pointy shoes.

Symptoms may include irritation of the skin around the bunion, and blisters may form more easily at the site. Pain may be worse when walking.

Bunions can lead to difficulties finding properly fitting footwear and may force a person to buy a larger size shoe to accommodate the width of the bunion. If the bunion deformity becomes severe enough, the foot can hurt in different places even without the constriction of shoes. It is then considered as being a mechanical function problem of the forefoot.

The exact cause is unclear.It can be due to a combination of internal and external causes. Proposed factors include wearing overly tight shoes, high-heeled shoes, family history, and rheumatoid arthritis. The American College of Foot and Ankle Surgeons states that footwear only worsens a problem caused by genetics.Diagnosis is based upon simple viewing of the foot, symptoms of pain, and by X-rays. Excessive low arch to a flat foot inward anke pronation of the foot also can cause increased pressure on the inside of the big toe that can result in a deformation of the medial capsular structures of the joint subsequently increasing the risk of developing a bunion.

The bump itself is due to the head of the metatarsal partly due to the swollen bursal sac or an osseous (bony) anomaly on the metatarsophalangeal joint. The larger part of the bump is a normal part of the head of the first metatarsal bone that has tilted sideways to stick out at its distal (far) end (metatarsus primus varus).

Bunions are commonly associated with a deviated position of the big toe toward the second toe, and the deviation in the angle between the first and second metatarsal bones of the foot. The small sesamoid bones found beneath the first metatarsal (which help the flexor tendon bend the big toe downwards) may also become deviated over time as the first metatarsal bone drifts away from its normal position. Osteoarthritis of the first metatarsophalangeal joint, diminished or altered range of motion, and discomfort with pressure applied to the bump or with motion of the joint, may all accompany bunion development. Atop of the first metatarsal head either medially or dorso-medially, there can also arise a bursa that when inflamed (bursitis), can be the most painful aspect of the process.

Bunions can be diagnosed and analyzed with a simple x-ray, which should be taken with the weight on the foot. The hallux valgus angle (HVA) is the angle between the long axes of the proximal phalanx and the first metatarsal bone of the big toe. It is considered abnormal if greater than 15–18°. The following HV angles can also be used to grade the severity of hallux valgus:^{[unreliable medical source?]}