This study investigates the effectiveness of a digital game--GraphoLearn (GL)--in supporting second-grade students who have persistent difficulties with acquiring accurate and fluent reading skills. The participants (N = 37) were randomly assigned either to a 6-week intervention including sessions with GL, in addition to school-provided support, or a control group receiving only school-provided support. The intervention took place at the students' homes and schools under the supervision of their parents and teachers. The results showed that the children who received the GL intervention developed significantly faster in word reading than the control group. Moreover, their reading development was significantly faster during the GL intervention compared with that of the follow-up period, which included only typical school-provided support. No transfer effects on reading fluency, reading comprehension, or spelling were found. Furthermore, the children who, according to the observations of their parents and teachers, showed higher cognitive engagement during the intervention had higher gains in word reading and sentence reading fluency than the children who appeared less cognitively engaged. Higher emotional engagement was related to increased playing time but not to larger gains in reading. The study indicates that a short digital game-based intervention training of letter-sound correspondences and word-level reading can give a boost to the reading development of struggling readers. Inspecting the engagement and in-game performance during gameplay provides important information that can be used for further development of the game to respond to the needs of the learners with severe difficulties.