<p dir="ltr">Vocabulary is a crucial element in second language learning. However, researchers in vocabulary acquisition express concerns about students’ successful acquisition of vocabulary (e.g., no significant gain after one semester of instruction (Clark & Ishida, 2005)) and the limited classroom instruction dedicated to vocabulary. In an effort to address these issues, the present study developed an online vocabulary learning system intended for use as homework, incorporating relevant theories, hypotheses, and empirical findings from existing literature and investigated its effectiveness employing a mixed-methods design.</p><p dir="ltr">For the quantitative component, students’ vocabulary gains were measured across three aspects of vocabulary knowledge (breadth/size, depth, and speed of access). A three-week experiment was conducted with students enrolled in the third-semester Japanese language course at a US Midwest institution. The final dataset included 54 students’ data. The experimental group (<i>n</i> = 28) utilized the new system to learn target words, while the control group (<i>n</i> = 26) used the current system employed in the course. The current system is also operated online and includes two types of exercises (i.e., listen-and-repeat and flashcards). ANCOVAs were employed to identify any significant differences between the groups, controlling for their pretest scores. Additionally, regression analyses were conducted to explore the relationship between the time the experimental group students spent learning new words using the new system and their outcomes, while also controlling for their pretest scores.</p><p dir="ltr">For the qualitative component, eight students from the same participant pool as the quantitative component participated in one-hour focus group discussions, conducted separately for the experimental and control groups.</p><p dir="ltr">The quantitative analysis revealed no significant differences between the groups; however, it was found that the time spent by the experimental group learning new words using the system significantly predicted two aspects of vocabulary knowledge. The qualitative data offered insights into potential explanations for the lack of significant differences between the groups, including the influence of students’ motivation on the experiment and the perceived difficulty level of the vocabulary exercises implemented in the new system. Based on the results of the present study, numerous suggestions are made for future development projects of similar systems and research.</p>