The last few decades have seen an increased interest in 'Mental Health Literacy (MHL)' which addresses knowledge and beliefs relating to mental health problems and is likely to hinder treatment-seeking behaviors. Although MHL research to date has explored various mental disorders, far too little attention has been paid to Internet Gaming Disorder (IGD) and Problematic Smartphone use (PSU). The objective of this study is to develop an MHL questionnaire for IGD and PSU by adopting the MHL approach in the previous literature and examine MHL of IGD and PSU in Korean teenagers by focusing on their ability to recognize addictions, and perceptions and help-seeking attitudes towards a person with addictions. The current study also aimed to compare Korean teenagers' MHL depending on low- and high-risk of IGD and PSU. A total of 169 teenagers aged 10 and 16 years were recruited from schools and children's centers in Seoul and completed the newly developed MHL questionnaire and screening questionnaires for IGD and PSU. The MHL questionnaire for IGD and PSU was designed as a vignette-based questionnaire that depicted behavioral characteristics of a severe problem with Internet and smartphone usage. Korean teenagers had a poor ability to recognize IGD and PSU and were more prone to seek parental help than professional help. The current study also showed that teenagers had better recognition of IGD than PSU and perceived that a person with IGD has a more difficult life than a person with PSU. Furthermore, a low-risk or high-risk group of IGD or PSU showed minimal differences in MHL. The current study provided empirical evidence to support a need to develop educational programs to improve Korean teenagers' MHL for both IGD and PSU.