This paper presents findings of an analysis of lexical errors caused by semantic similarity and discusses pedagogical implications for teaching of vocabulary in Korean as a foreign or second language (KFL/KSL). It has been reported that lexical errors are caused most frequently or mainly by confusion based on semantic similarity. These lexical errors are related to a lack of knowledge of the conceptual differences between the competing words. KFL/KSL learners in general feel great difficulties in differentiating lexical items with similar meaning, and the difficulties are primarily associated with the definitional concepts and semantic restrictions (Sohn, 1986: 499). This study has attempted to refine the working definition of ‘errors of semantic similarity’ and present some examples of lexical errors of semantic similarity by category, along with attempts to explain the cause of such errors. The lexical errors presented in this study are part of 310 lexical errors identified out of 167 examination papers from three tertiary institutions. The study then discusses some pedagogical implications and strategies to deal with such problematic lexical items.