The deal would see the pair acquire a 10 per cent stake in Citic for HK$34.4 billion ($4.4 billion) and invest another HK$45.9 billion in convertible preferred shares, which could ultimately double the size of their stake in the Chinese firm.
By investing in politically-connected Citic, the firms would likely gain easier access to highly regulated Chinese business sectors, including resource development, logistics and real estate.
"Itochu Group has always weighed emphasis in Asia, but the China market has been considered as most important of all, and has been actively expanding trade and construction of strategic cooperation with leading local businesses," Itochu said in a statement.
"(The three firms) are highly complementary with each other and match well either in terms of business layout or scale, and the combination of strength will help to bring about more synergies from the strategic cooperation...in the ever-expanding China and Asia markets."