Learning a foreign language is a sophisticated and time-consuming procedure filled with challenges and varied emotions, requiring an examination of grit and emotional factors in achieving successful outcomes. However, limited research has explored how grit predicts success, its connections with common emotions like enjoyment and boredom, and how these psychological elements influence a learner’s vocabulary knowledge. To this end, 100 Iranian EFL learners, ages 18 to 22, with intermediate English proficiency and approximately 3 to 7 years of English learning experience, were selected through a convenience sampling method, consisting of 50 females and 50 males. The first questionnaire used consisted of 12 items adopted by the participants to measure the grit scale. The second questionnaire consisted of 36 items given to the participants to measure the emotion scale. The third questionnaire was about the General ESL Vocabulary MCQ Test to estimate participants’ vocabulary knowledge. The study's findings indicated a significant positive relationship between emotion and vocabulary knowledge (r(102) = 0.34, p = 0.000), but not between other variables. Besides, it was found that positive emotion situations can predict vocabulary knowledge (t = 3.60, p = 0.001, Beta = 0.34), but grit does not. The results suggest implications for enhancing students’ language learning.