Background <p>Negative body image is a prevalent issue among young individuals. Loneliness induces negative cognitive biases in individuals, which contribute to more negative self-evaluations and social perception. However, the role of loneliness as a potential risk factor for negative body image, along with the neural mechanisms underlying this relationship, remains insufficiently explored.</p> Methods <p>This study used resting-state functional network connectivity to investigate brain network connectivity patterns associated with loneliness in a large sample of young individuals (<i>N</i> = 1,358, 67.53% female, <i>M</i>_age = 19.03 years). Additionally, we explored potential neural connections through which loneliness may influence negative body image.</p> Results <p>Increased functional connectivity between the default mode network (DMN) and the frontoparietal network (FPN), and decreased connectivity between DMN and the subcortical network (SCN), and between FPN and SCN, were correlated with higher loneliness. The results were validated through machine learning and sensitivity analyses. Young individuals experiencing higher loneliness tended to have more negative body images. The DMN-FPN and FPN-SCN connectivity mediated the relationship between loneliness and negative body image, particularly in the facial appearance, fatness, and shortness dimensions.</p> Conclusions <p>Our findings provide new evidence of atypical brain activity in lonely individuals, suggesting aberrant connectivity patterns in self-recognition (i.e., DMN), social perception (i.e., FPN), and emotional processing (i.e., SCN) networks. This disturbance may affect individuals’ body image. These findings offer new insights into the neural mechanisms underlying loneliness and its psychological impact on young populations.</p>