This study aims to investigate the effects of semantic clustering on Korean secondary school students’ English vocabulary learning. The present study was conducted with 56 secondary school students in South Korea. In order to determine if and how semantic clustering affects second language learners, the participants were divided into two groups: a semantically related (SR) word group and a semantically unrelated (SU) word group. The two comparison groups were taught the same 25 vocabulary items in different clusters (SR vs. SU). In the experiment, vocabulary test scores were gathered and analyzed three times: on a pre-, post-, and delayed-test. In the results, scores from the SU word group significantly outperformed the SR word group on the post-test, but on the delayed-test. Findings suggest that presenting new L2 words in SU sets is more effective than SR sets in the short term with Korean EFL secondary school students.