A retired US medical school professor and AIDS researcher has gone missing during a marathon swimming race down New York’s Hudson River.

Authorities say 67-year-old Dr Charles van der Horst was reported missing just after 3pm on Friday near the George Washington Bridge. Police say a man who was watching from the river bank saw van der Horst disappear and called 911.

The New York City Police Department resumed searching for van der Horst on Saturday after suspending the search late on Friday.

Charles can der Horst: There one minute, gone the next. Photo: University of Colorado

Van der Horst was taking part in a multi-stage 193-kilometre race down the Hudson called the Eight Bridges Race. Organisers cancelled the seventh and final stage of the race.

New York Open Water, the organisation that runs the race, said in a statement,

“Our thoughts are first and foremost with the Van Der Horst family and we ask that all respect their wish for privacy as they mourn this tragic loss”.

The group said safety protocols were in place and police were escorting the swimmers.

Van der Horst retired from the University of North Carolina, Chapel Hill medical school, where his career encompassed clinical medicine, teaching and research.

He has served as a consultant helping to implement AIDS treatment and prevention programs in South Africa and as a volunteer physician at a free clinic in Raleigh.

-AAP