The objective of this study was to investigate the influence of the popular Duolingo App on learning English in Thailand. Parts of the Duolingo English course for Thai speakers were used in a classroom-based intervention with two sections of English for Professional Purposes courses at a Thai university (n = 128). The students were split into two groups of users (n = 70) and non-users (n = 58), and a pre-test/post-test assessment of verb tense knowledge was conducted. The results showed that the Duolingo users had a significantly higher mean improvement in verb tense knowledge. Multiple regression showed that frequency of Duolingo use, perceived achievement, and perceived enjoyment positively influenced performance among the user group. Moderation analysis also showed that prior use of Duolingo partially moderated the relationships of level completion and achievement on score improvement. The implication is that Duolingo is a potentially useful supplement to classroom learning although its effects may change over time as users become habituated.