This study investigated the role of interactive output tasks in developing EFL learners’ vocabulary knowledge. The participants were 103 elementary female Iranian EFL learners who were randomly divided into three groups: input-only, input-output-no-interaction, and input-output-interaction. After all participants took a placement test and a vocabulary pretest, the input-only group was exposed to input tasks, while the other two groups received both input and output tasks with or without interaction. Then, all the participants took a vocabulary posttest. The results of ANOVA and Kruskal-Wallis tests showed that the participants in both the input-output-no-interaction group and the input-output-interaction group outperformed the ones in the input-only group in the vocabulary posttest (in both the overall vocabulary test and in the productive vocabulary section). Moreover, the results of the t-test and the Mann-Whitney test revealed that the participants in the interaction and no-interaction groups performed similarly on both the overall vocabulary posttest and the productive vocabulary section. The findings of this study support the idea that output is a facilitative factor for the acquisition of L2 vocabulary and, specifically, productive vocabulary development. The results also suggest that both interactive and non-interactive output-plus-input tasks can lead to higher achievement in vocabulary knowledge compared to the input-only condition lacking output tasks.