The impact of vocabulary knowledge on speaking is not often investigated in EFL learning contexts. This study investigated the relationship between students’ vocabulary knowledge and their speaking ability. The study aimed at determining the extent to which there is a correlation between their vocabulary knowledge and their speaking proficiency. This correlational study used a mixed-method design to collect data from 31 1re A students in Lycée Communal Rimvougré in Ouagadougou. A vocabulary test, a speaking test, and a semi-structured interview were the tools used to collect data. The findings from the vocabulary test and the speaking tests revealed a high positive correlation. The semi-structured interview contributed to explaining the nature of the correlation. It reveals that the participants perceived vocabulary as the main obstacle to their speaking proficiency. The study indicates that vocabulary knowledge accounts for more than half of the variance in their speaking proficiency, suggesting that increased vocabulary knowledge is associated with better speaking proficiency. Therefore, educators should help learners enhance their vocabulary knowledge and improve their speaking ability.