Marx won the award for the first time, and seven years after his only other nomination.
Also shortlisted were Boks teammates Pieter-Steph du Toit, the award winner in 2019 and 2024, and Ox Nche, the first prop to be nominated in the award's 24-year history.
Also nominated was France winger Louis Bielle-Biarrey, whose eight tries in the Six Nations were the most in a single tournament in 100 years.
The 31-year-old Marx was the joint leading try-scorer with four in the Rugby Championship that South Africa won. His five Test tries this year improved his record to 26 in 86 Tests; the only forward in South Africa's all-time top 10.
He's also the fourth Springbok to win the men's award after Du Toit, Schalk Burger (2004) and Bryan Habana (2007).
New Zealand lock Fabian Holland was named the breakthrough player of the year.
Ex-All Blacks mentor John Mitchell was Coach of the Year after guiding England to Women's Rugby World Cup success, though that award was announced in September.
Canada's Sophie de Goede took the World Rugby Women's 15s Player of the Year, and breakthrough player went to Braxton Sorensen-McGee of New Zealand.
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