"Hanashite Honyaku" will be a free application that can be used on smartphones and tablet computers with the Android operating system, DoCoMo said.
Customers will also be able to call landlines using the service, it said, adding that voice-to-text readouts will soon be available in French, German, Indonesian, Italian, Portuguese, Spanish and Thai.
"We hope that with this application, our subscribers will be able to widen the range of their communication," a company spokeswoman said.
However, she conceded the service does not offer perfect translations and has trouble deciphering some dialects.
DoCoMo also said it has launched a separate service that lets users translate menus and signage using the smartphone camera.