Young adults are at elevated risks of hikikomori (severe social withdrawal) under the COVID-19 pandemic, which could lead to psychological distress and suicidal ideation. The present study aimed to examine the associations among hikikomori, suicide stigma, suicidal ideation, and help-seeking behaviors among young adults in Hong Kong. This study recruited a large sample of 2,022 young adults in Hong Kong via an online survey at the end of 2021. The participants completed the Hikikomori Questionnaire and validated measures on psychological distress, suicide stigma, and suicidal ideation severity and report their help-seeking behaviors. Multivariate analysis of variance was conducted to compare the profiles of the hikikomori groups. Path analysis evaluated the effects of hikikomori and suicide stigma on occurrence and severity of suicidal ideation and their associations with help-seeking behaviors. Hikikomori showed significant and positive indirect effects on prevalence and severity of suicidal ideation via psychological distress. Glorification positively predicted hikikomori and suicidal ideation severity among suicidal persons. Hikikomori was associated with reduced help-seeking. Isolation and suicidal ideation were associated with greater barriers to seek help among the non-help-seekers. Perceived helpfulness of sought help was negatively associated with hikikomori and suicidal ideation among the help-seekers. The present findings demonstrated greater prevalence and severity of suicidal ideation and less help-seeking in young adults with hikikomori. Suicide stigma showed differential associations with hikikomori, suicidal ideation, and help-seeking behaviors.