<p>Loneliness and non-suicidal self-injury are significant mental health concerns during adolescence, yet less is known about how they develop together across middle school and whether changes in one are followed by changes in the other. The present study addressed this issue using six waves of longitudinal data collected over three years from 3,800 Chinese middle school students (Mage = 12.38, SD = 0.51; 49.4% female). Across middle school, loneliness tended to increase, whereas non-suicidal self-injury tended to decline. These developmental trends were also linked: higher initial loneliness was associated with a slower decline in non-suicidal self-injury, whereas higher initial non-suicidal self-injury was associated with steeper increases in loneliness. At the within-person level, elevated loneliness predicted subsequent increases in non-suicidal self-injury, whereas fluctuations in non-suicidal self-injury did not predict subsequent increases in loneliness. Sensitivity analyses further suggested that this within-person association was more consistent with a direction from loneliness to non-suicidal self-injury, although the data could not clearly determine whether the association was primarily lagged or contemporaneous. Together, these findings suggest that loneliness and non-suicidal self-injury are linked across both long-term developmental trajectories and short-term within-person fluctuations, while also highlighting the more prominent role of loneliness in their relationship during middle school.</p>