This study investigates the effects of multilingualism and the degree of proficiency in different languages on vocabulary knowledge among Spanish language learners. Participants completed a 160-word meaning recall test designed to measure their knowledge of the 8,000 most frequent Spanish words and a self-assessment questionnaire on their multilingual profile. The results show that students with high multilingual profiles (knowing more than three languages) demonstrated greater vocabulary knowledge than learners with low multilingual profiles (knowing three or fewer languages), especially for words in low-frequency vocabulary ranges. However, the positive impact of multilingualism on vocabulary knowledge is only significant among learners with a high proficiency in Spanish (C1-C2 level), suggesting that identifying as highly proficient in multiple languages is advantageous only when learning vocabulary at higher proficiency levels. These results corroborate the benefits of multilingualism for fostering vocabulary development in a foreign language, while offering a nuanced picture of how additional languages (LX) are acquired by multilingual learners.