Language educators and researchers frequently advocate two things in language acquisition: vocabulary learning and reading for pleasure. Vocabulary knowledge plays a crucial role in language mastery and extensive reading is deemed extremely fitting for vocabulary learning. For one, it is considered a pedagogically efficient approach as both reading and vocabulary acquisition can occur simultaneously. Sustained Independent Reading (SIR) is a vocabulary knowledge development intervention based on the belief that self-selection motivates readers to read, broadly and independently, over a sustained period of time. The present study was carried out in a public university in Malaysia over the course of one semester, involving two groups of 25 Malaysian participants each. The first group (Experimental) experienced 10 weeks of SIR while the second group (Control) did not. The participants were enrolled in the same remedial English proficiency course at the university and were matched in terms of vocabulary size. Results from the pre-, post- and delayed post-tests show greater lexical gains for the Experimental group with respect to vocabulary size, indicating that SIR is an effective method for motivating English language learners to read (broadly, independently and over a sustained period of time) and more importantly, for vocabulary knowledge development. This study's findings are relevant to stakeholders of English language education, specifically with regards to promoting extensive reading at the tertiary level, shaping Malaysian ESL policy in higher education, and providing direction for future related research.