Rio said the partnership "will explore a range of technologies for decarbonisation across the entire steel value chain from iron ore mining to steelmaking, including integrating Rio Tinto's iron ore processing technology and Posco's steelmaking technology".
Posco has said it wants to decarbonise steelmaking by 2050 by using green hydrogen instead of metallurgical coal, and in March said it would invest Won 10 billion (US$8.7 million) to develop the technology.
The MOU means Rio has agreements in place with three of Asia's largest steel producers, having already signed MOUs with China Baowu Steel Group - the world's top steel producer - and Japan's Nippon Steel Corporation.
Rio is hoping the agreements will help deliver a 30% reduction in the carbon intensity of steelmaking by 2030.
Rio is not alone among the mining majors in its quest to reduce emissions from the steelmaking process. BHP signed a five-year MOU with Baowu in November along similar lines to Rio's partnerships.