Iron ore shipments will rise to 133 million tonnes a year from 80 million tonnes, Mitsui said in a statement.
The investment will expand a port in Cape Lambert in Western Australia that is used to ship iron ore produced at the Robe River joint venture.
Rio Tinto owns 53 percent of the venture and Mitsui 33 percent.
The total cost of the planned expansion is 300 billion yen and the joint venture participants will share the cost on pro-rata basis. It is due to be completed by the end of 2013.
"Through this expansion, we hope to meet rising iron ore demand in the medium to long term," Mitsui said in a statement.
Mitsui's shares closed up 0.7 percent at 1,325 yen ahead of the announcement.