Managing director Dominic Booth said the company had "fulfilled our long-standing objective" of running the franchise as a 60:40 joint venture and the deal will deliver "significant improvements" for Greater Anglia's customers.
Abellio won the Greater Anglia franchise again last August, having first operated it from February 2012.
The company said: "Abellio and Mitsui have a proven track record of working together, having first entered into a joint venture to bid for the West Midlands rail franchise in 2016, along with East Japan Railway Company."
"Following this process, Abellio felt that Mitsui would be the best partner to help it deliver its ambitious programme to transform the Greater Anglia franchise."
It did not say how much the deal – which is subject to final regulatory approvals – is worth.
Abellio is overseeing a £1bn Bombardier contract to deliver new train carriages as part of a £1.4bn boost to rail services across East Anglia, announced last year by the Department for Transport (DfT).
It follows National Express' sale of the C2C franchise to Italian firm Trenitalia, in a deal worth £70m and marking the transport giant's exit of the UK rail market.
And like the National Express deal, this news has led to union criticism, with the Rail, Maritime, Transport (RMT) union saying Britain's transport assets are being "knocked down like Del Boy at some dodgy car boot sale".
General secretary Mick Cash said: "Following on from National Express sale of the c2c franchise to Trenitalia, this latest sell off by Abellio of a chunk of the Greater Anglia franchise to Mitsui shows that the Britain's rail network is up constantly for grabs making a mockery of the expensive DfT franchising process.
"The checks and balances for both passengers and the taxpayer, which the DfT claims are enshrined in its multi-million pound franchising programme, are clearly lacking when the winning bidder can simply walk away, share out its responsibilities and choose its replacement whenever it sees fit."