Background: Both Internet Gaming Disorder (IGD) and Internet Addiction (IA) have been associated with diverse psychopathological symptoms. However, how the two conditions relate to each other and which is more strongly associated with psychopathology remain unclear. Objective: This study aimed to examine the association between IGD and IA and to compare the strength of their associations with various types of psychopathological symptoms. Methods: This cross-sectional study surveyed three independent samples of Chinese adolescents: the first sample (S1) comprised 8,194 first-year undergraduates at a comprehensive university in Chengdu; the second sample (S2) comprised 1,720 students from a high school in Hangzhou; and the third sample (S3) comprised 551 inpatients aged 13–19 years recruited from two tertiary psychiatric hospitals in Hangzhou and Chengdu. IGD was defined as a score ≥ 22 on the Internet Gaming Disorder Scale–Short Form (IGDS9-SF), whereas IA was defined as a score ≥50 on Young’s 20-item Internet Addiction Test (IAT-20). Symptoms of depression, anxiety, psychoticism, paranoid ideation, and attention-deficit/hyperactivity were assessed using internationally validated scales including Patient Health Questionnaire-9 (PHQ-9), Generalized Anxiety Disorder-7 (GAD-7), psychoticism and paranoid Ideation Subscales of the Symptom Checklist 90 (absence for S2) and Adult ADHD Self-Report Scale (absence for S1), though online surveys in S1 (October 2020) and S3 (January 2022 to February 2025) and via an offline survey in S2 (March 2024). Results: Prevalence estimates (95% CIs) of IGD were 4.8% (4.3–5.2%) in S1, 15.8% (14.0–17.5%) in S2, and 32.3% (28.4–36.2%) in S3, whereas prevalence estimates of IA were consistently higher across samples, ranging from 7.3% (6.8–7.9%) in S1 and 18.8% (17.0–20.6%) in S2 to 45.9% (41.8–50.1%) in S3. IGDS9-SF and IAT-20 were moderately correlated (Pearson’s r = 0.51–0.57, all p