Tottoko Hamtaro: Tomodachi Daisakusen Dechu is a fortune-telling game released in Japan for the Game Boy Color in 2000. Though it has the appearance of a virtual pet game, the game contains no pet-raising elements nor direct ways to interact with Hamtaro. Rather, the game asks the player to fill out a notebook with information (full name, nickname, gender, birth date, and blood type) about themselves, their family, and their friends. This information is then used to pseudo-randomly generate predictions about the person – which Ham-Ham they're most like, their personality traits, their romantic or platonic compatibility with other people that have been entered, etc. – creating the appearance of fortune-telling. The game also has a real-time element, through which Hamtaro will occasionally speak to you of his own accord. Notebook entries can also be traded with other people via the Game Boy Color's infrared feature.

It was the ninth best selling game on the Game Boy Color platform in Japan, with 343,950 copies sold. It would later be the foundation for Tomodachi Collection (and, by extension, the Tomodachi series) – in a Japanese Iwata Asks interview, Ryutaro Takahashi revealed that the development of that game began with Yoshio Sakamoto stating: “I want to make a version [of Tomodachi Daisakusen Dechu] that adult women could play.” This led to the game's inception under the working title Otona no Onna no Uranai Techō (大人のオンナの占い手帳, lit."The Adult Woman's Fortune-Telling Notebook").