No studies have investigated the associations between established plant-based diet indices and the metabolic syndrome (MetS). We evaluated the associations between an overall plant-based diet index (PDI), healthy PDI (hPDI), unhealthy PDI (uPDI) and the MetS in a nationally representative sample using data from 14 450 Korean adults (≥19 years) in the Korea National Health and Nutrition Examination Survey 2012–2016. Dietary intakes were assessed by a semi-quantitative FFQ. In the PDI, all plant foods received positive scores. In the hPDI, only healthy plant foods received positive scores. In the uPDI, only unhealthy plant foods received positive scores. All indices reverse scored animal food intake. Multivariable logistic regression models were used to examine the associations between three PDI and the MetS by sex, adjusting for potential risk factors. A total of 23·3 % of Korean adults had the MetS. In the overall study population, individuals in the highest quintile of uPDI had greater odds (OR 1·54, 95 % CI 1·28, 1·86, Ptrend < 0·001) of the MetS than those in the lowest quintile. Higher uPDI score was associated with higher odds of hypertriacylglycerolaemia in men and abdominal obesity, high fasting glucose and hypertriacylglycerolaemia in women. No significant associations were observed between PDI, hPDI and the MetS. Greater adherence to unhealthy plant-based diets was associated with greater odds of the MetS and its components suggesting the importance of the quality of plant-based diet in South Korean adults. Sex differences may be considered when recommending plant-based diets for the prevention and management of metabolic diseases.