Stigma against people living with HIV (PLHIV) fosters depression and negatively impacts the quality of life in PLHIV and is a barrier to the whole process of treatment. This study aimed to identify the levels of knowledge and stigmatizing attitudes toward HIV infection among Korean men in their 20s and the factors influencing them. A cross-sectional design was used. Two hundred and eight Korean men in their 20s responded to self-report questionnaires that included items on knowledge and stigmatizing attitudes about HIV infection, subjective norms for safer sexual behaviors (SSBs), participants’ HIV-related characteristics, sex-related characteristics, and general characteristics. The mean score (±SD) for knowledge was 13.9 (±5.28) and for stigmatizing attitudes was 64.1 (±11.42). In quantile regression analysis, exposure to HIV-related information was a significant factor for knowledge in every quantile, and experience of HIV education was a significant factor in the 50th quantile. Experience of meeting PLHIV was a significant factor for stigmatizing attitudes in the 25th quantile, and subjective norms for SSB were a significant factor for stigmatizing attitudes in the 25th and 50th quantiles. Findings suggest the need for intervention to improve young Korean men’s knowledge, as well as intervention focusing on norms for SSB, to prevent stigmatizing attitudes.