This paper focuses on the design, implementation and evaluation of an online game for elementary and middle school mathematics. Its aim is twofold: (a) the development of the prototype of a flexible and adaptable computer game, and (b) the evaluation of this prototype, as to its usability and technical aspects. The particular computer game was created in an attempt to facilitate the teaching of mathematics, a subject that is often regarded as complicated by students of all ages. Apart from the game, an administration website was also constructed, so that the educator can configure the game, without that requiring any programming skills. More specifically, the educator can use the administration website in order to alter several of the game's parameters, such as the content and total number of its questions.The game was evaluated in real school settings, both through a pilot study with 12 students and a longterm intervention with 37 students that lasted 14 weeks. The results indicated that the students' opinion about the game was positive, and suggest that with some extensions the game could be used as an effective learning tool. Finally, some corresponding conclusions and future improvements to the game are being discussed on the basis of the findings.