Unite and the Public Commercial Services unions have signed an agreement to work together in representing IT workers at Fujitsu.
They have previously worked together at HP.
The unions said that since outsourcing forms a major part of Fujitsu's business, they will also work together to recruit workers transferring into the company from other organisations.
Unite general secretary Len McCluskey said: "This agreement recognises the nature of Fujitsu's business and ensures that the unions cooperate to best represent their members within the company.
"Whether workers are transferred in, or already work for the company, this deal ensures they have the best possible representation."
The unions will carry out joint campaigns locally and nationally, and share information to build their union organisation at Fujitsu.
PCS general secretary Mark Serwotka said: "The government's obsession with austerity has bled into private companies and many of them are using the recession as an opportunity to cut staff and wages to make ever higher profits.
"We look forward to building on the excellent links we already have with Unite in Fujitsu, as well as elsewhere in the private and public sectors, to ensure workers can stand together to improve their working lives and living standards."