The compact device, which will be released in Japan next month, is designed to fit inside a card holder.
It is part of a trend to offer so-called companion mobile devices to those who already own larger smartphones.
The phone weighs 47g (1.6oz) and features an electronic-paper touchscreen, similar to e-books.
It will have limited functionality, with calls, texts and a net connection but no camera or facility to work with mobile apps.
The phone, dubbed Card Keitai KY-01L will be made by electronics firm Kyocera.
NTT Docomo describes the mobile as the "world's thinnest" but that claim is likely to be disputed by other phone makers.