This study investigated the effect of derivational suffix instruction on vocabulary knowledge. Fifty-eight high school learners were recruited to participate in the study. The learners were divided into control (n = 29) and experimental (n = 29) groups. The experimental group received a Fundamental English II course with an emphasis on derivational suffix instruction, whereas the control group only received a Fundamental English II course. Two receptive and productive derivational suffix tests were designed and validated to measure high school learners' suffix knowledge. A questionnaire was also administered to explore the experimental participants' perceptions of derivational suffix instruction in facilitating vocabulary knowledge. Descriptive and inferential statistics were conducted to analyse the quantitative data, including pair-sample t-tests and independent-sample t-tests. The results showed that high school learners in both groups performed significantly better on the posttest than on the pretest. The results also revealed that experimental learners significantly outperformed their control counterparts. These findings suggest the efficacy of derivational suffix instruction on English vocabulary knowledge in Thai high school learners. In addition, the questionnaire analysis indicated that high school learners had positive perceptions towards derivational suffix instruction as a method to improve their vocabulary knowledge. Together, the current findings highlight the importance of derivational suffixes in vocabulary acquisition and development. Other relevant implications and suggestions for future studies are also discussed.