Marius Stephanus Barnard (November 3, 1927 – November 14, 2014) was a South African cardiac surgeon and inventor of critical illness insurance.

Barnard was a member of the team headed by his brother Christiaan Barnard that performed the world's first human-to-human heart transplantation in 1967.[3] Specifically, he was one of the surgeons who removed the heart from donor Denise Darvall at Groote Schuur Hospital. After a 2009 documentary film Hidden Heart suggested that Hamilton Nakiremoved the donor heart, Barnard was quoted as describing the film as "rubbish, a joke, it’s a total distortion of the facts" and as stating that Naki was at the time "in his bed, about 8 km away from Groote Schuur".

Barnard was motivated by the financial hardship he saw his patients suffer after he had treated their critical illnesses to convince the South African insurance companies to introduce a new type of insurance to cover critical illnesses. Barnard argued that, as a medical doctor, he can repair a man physically, but only insurers can repair a patient's finances. 

On 06 August 1983 the first critical illness insurance policy was launched.

Barnard was a member of the South African parliament between 1980 and 1989, for the Progressive Federal Party - one of the few political parties that opposed apartheid. He later acted as a technical consultant for Scottish Widows. Barnard has received many awards for his contributions to medicine and humanity, and was voted in the top 25 most influential people in the field of health insurance and protection.

He died on November 14, 2014 after battling with prostate cancer.