Speech sound disorder (SSD) is one of the most common communication disorders in children, which must be diagnosed and treated in childhood to prevent its negative psychological, occupational, and social consequences in adulthood. The use of serious games is one technology that could help treat SSD. This study aims to design and evaluate the usability of a digital game for 3–6-year-old children. This developmental study was carried out in three main stages: determining information requirements, designing and developing a digital game, and evaluating the usability of the designed game. In the first stage, the consonants, syllables, words, and sentences that should be used in the game were determined through focus group sessions and a questionnaire completed by 30 speech-language pathologists (SLPs) in two Delphi rounds. In the designing and developing stage, the game design document was first written, and then sketches and wireframes were drawn based on this document. Then, the game was developed in the second phase using Java programming language. In the third stage, the usability of the designed game was evaluated by six experts and 22 SLPs using Nielsen's usability heuristics and the Post-Study System Usability Questionnaire (PSSUQ), respectively. Also, the usability of the game was evaluated by ten children. To determine the information requirements, SLPs confirmed 23 consonants, 138 syllables, 564 words, and 69 sentences to teach children the game. According to Nielsen's usability heuristics, the designed game had 23 unique usability problems with a mean severity of 2.29. "User control and freedom," with five usability problems, was the most frequently identified problem, and "error prevention," with a mean severity of 3.83, was the most severe problem. The results of the SLPs’ evaluation also showed that the "interface quality" of the designed game obtained the highest score, with a mean score of 6.25 out of 7. Also, 100% of children stated that the game was designed beautifully, and 90% of children also liked the game. A serious game designed for children with SSDs achieved satisfactory results in usability evaluation. Therefore, this smartphone-based game can be easily used among these children to provide fun therapy sessions under the supervision of parents.