This study aimed to test the differences of weight-related self-stigma and mental health conditions between overweight (OW) and non-OW children in Hong Kong. The correlations between weight-related self-stigma and mental health conditions were also investigated. Children aged 8 to 12 years (n = 367; 198 boys) completed questionnaires on weight-related self-stigma (Weight Bias Internalization Scale [WBIS] and Weight Self-Stigma Questionnaire [WSSQ]) and mental health conditions (Brief Symptom Rating Scale-5 [BSRS-5]). Compared with non-OW children (n = 241; 143 boys), OW children (n = 114; 55 boys) had higher weight-related self-stigma in the WBIS (26.49 ± 8.68 vs. 21.58 ± 7.54; p < 0.001) and WSSQ scores (26.36 ± 8.98 vs. 21.91 ± 8.71; p < 0.001). No significant difference was found between OW and non-OW children in mental health conditions as reflected by the BSRS-5 score (4.29 ± 4.35 vs. 4.44 ± 4.16; p = 0.761). BSRS-5 was significantly associated with the WBIS. OW children tended to have a higher level of self-stigma; those having a higher level of weight-related self-stigma presented with more mental health problems.