According to Asiajin, the two companies have already launched their first joint-product - Mixixmas, a Christmas themed app that publishes both to Mixi and Twitter.
Mixi has a reported 22 million users, while Twitter has about 16 million.
The social networks will work together on special occassions, but don't appear to have plans to work together on an everyday basis, reports The Next Web.
Twitter and Facebook both gained popularity due to their effectiveness during the Japanese earthquake earlier this year. Twitter and Mixi will also be working together to provide communication enabling services during emergencies and disasters.