Loneliness may lead individuals to spend more time on the internet and increase the likelihood of experiencing internet-use disorders. Similarly, individuals with internet-use disorders may feel lonelier. In the present systematic review and meta-analysis, pre-registered in PROSPERO (CRD42023390483), we quantified associations between internet-use-disorder symptoms (e.g. internet gaming disorder and online gambling disorder) and loneliness. We searched PubMed, Web of Science, and an institutional database aggregator for references that compared degrees of loneliness in groups of individuals with and without symptoms of internet-use disorder. Means and standard deviations of loneliness, or alternatively, odds ratios, were transformed into Cohen's d for statistical pooling through a random-effects model. After screening 2,369 reports, we extracted data from 23 studies. The total number of individuals across the studies was 36,484. Participants were between 13 and 30 years of age (median 20). The pooled difference between those with and without internet-use-disorder symptoms yielded a standardized effect (Cohen's d) of 0.53 (95% CI 0.35-0.7). While heterogeneity was high, there was no indication of publication or small sample biases. Similar effect sizes were found when limiting to specific types of internet-use disorder symptoms. Moreover, meta-regressions did not show an effect of age, sex, or sample size. Individuals with symptoms of internet-use disorders scored 49.35 (43.84-54.85) points on the UCLA-Loneliness scale on average, compared to 43.78 (37.47-50.08) in individuals without symptoms of internet-use disorders (Standardized Mean Difference: 5.18, 95% CI = 2.05-8.34). Individuals with internet-use-disorder symptoms experience greater loneliness. The effect appears moderately sized.