Smartphones are a necessity for many people; however, problematic smartphone use (PSU) may negatively influence people’s mental health. Using multivariate linear regression analysis, the study examined the associations of sexual minority stressors [namely perceived sexual stigma from family members, sexual orientation microaggressions (SOMs), and internalized sexual stigma] and gender nonconformity with PSU severity as well as the associations of PSU with depression and anxiety in young adult lesbian, gay, and bisexual (LGB) individuals. This cross-sectional survey study recruited 1000 young adult LGB individuals (500 men and 500 women). PSU severity was assessed using the Smartphone Addiction Inventory. The experiences of perceived sexual stigma from family members, SOMs, and internalized sexual stigma and the levels of gender nonconformity, depression, and anxiety were assessed. The results indicated that perceived sexual stigma from family members, SOMs, internalized sexual stigma in the dimensions of social discomfort and identity and gender nonconformity were significantly associated with PSU severity in LGB individuals. Moreover, PSU was significantly associated with depression and anxiety in LGB individuals. The findings highlight the significance of developing strategies for the prevention and early detection of PSU and sexual minority stress in LGB individuals.