In the United States, 16.6 million adults are unable to comprehend sentences or read common documents, thereby limiting their ability to participate in a literate society. Given the consequences of low literacy, we conducted a single-case study with adult multilingual learners to test the effectiveness of a game app, Codex: The Lost Words of Atlantis, on word decoding. The analyses of the pilot study showed moderate to large effect sizes on phonological decoding of taught and untaught words and large and statistically significant increases from pre-test to post-test on five distal reading measures. These results suggest the app may support adult multilingual learners in learning to decode common English words.