This research aimed to test relationship between loneliness with problematic internet use in young adults who are social media users. When the individual's expectations for relationships with others are not consistent with reality, the individual will experience loneliness. The internet is one of the ways to have a social interaction and reduce feelings of loneliness. Feeling comfortable using internet for social interaction is a characteristic of individuals with problematic internet use. This research is a quantitative research with correlation techniques. Participants in this research were 508 young adults aged 18-40 years consisting of 133 men and 375 women in Indonesia. This research uses the Generalized Problematic Internet Use Scale 2 which was arranged by Reinaldo & Sokang (2016) and UCLA Loneliness Scale Version 3 which was arranged by Fatriani (2019). Hypothesis test results using Pearson Correlation showed r (508) = 0.432, p = 0.000 < 0.05. These results indicate that there is a significant and positive relationship between loneliness with problematic internet use in young adults who are social media users.