Mr Walsh was made a non-executive director by the Mitsui board at a meeting in Tokyo on Wednesday. His appointment is subject to Mitsui shareholder approval at its annual meeting in June.
The appointment is a huge slap in the face for Rio Tinto because Mitsui is Rio's joint venture partner in the Robe River iron ore project in Western Australia.
Mr Walsh now finds himself in the unusual position of helping to oversee Mitsui's 33 per cent interest in the Robe River mine, which is operated by Rio.
His appointment to the board of Mitsui was announced to the Tokyo Stock Exchange on Wednesday afternoon.
Mitsui has been involved in Australia's iron ore industry since the 1960s. Apart from its interest in Robe River, Mitsui has a 7 per cent stake in four iron ore projects with BHP Billiton.
Mitsui also has extensive interests in seven coal mines in Queensland and NSW, which produce thermal and coking coal.
Mitsui also has a 20 per cent equity stake – through its shareholding in BHP Billiton Mitsui Coal – in the Poitrel and South Walker Creek mines in Queensland's Bowen Basin.
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