This study examines the relationship between metacognitive regulation and the acquisition of passive
vocabulary knowledge among Malaysian adult ESL learners. Metacognitive regulation involves decisions
about planning, monitoring, or/and evaluating the best ways to acquire English vocabulary. Two entities
make up metacognitive regulation in this study namely selective attention such as making notes of words
which seem important, and self-initiation such as reading other English reading materials besides textbooks
to expand one’s vocabulary knowledge. The metacognitive regulation level of the ESL learners is analyzed
using the Vocabulary Learning Questionnaire. Simultaneously, their passive vocabulary knowledge is
assessed using the Vocabulary Levels Test. Passive vocabulary knowledge is usually defined as what one
needs to know about a word in order to use it in reading and listening. 360 university students aged between
18 to 21 years old were involved. Though metacognitive regulation is not that preferred by the respondents, it
positively and significantly correlates with passive vocabulary knowledge. Further discussion focuses on the
significance of metacognitive regulation in vocabulary acquisition. This paper concludes with a discussion on
the pedagogical implications of these results.