BACKGROUND The objective of the current study was to examine social functioning among adolescents and young adults (AYAs) within the first 2 years after a cancer diagnosis and compare their scores with population norms and identify trajectories of social functioning over time and its correlates. METHODS A multicenter, longitudinal study was conducted among 215 AYA patients with cancer aged 14 to 39 years. A total of 141 patients completed a self‐report measure of social functioning within the first 4 months of diagnosis and again at 12 months and 24 months later. RESULTS AYA patients with cancer were found to have significantly worse social functioning scores around the time of diagnosis (52.0 vs 85.1; P<.001), at the 12‐month follow‐up (73.1 vs 85.1; P<.001), and at the 24‐month follow‐up (69.2 vs 85.1; P<.001) when compared with population norms. Significant improvements in social functioning from baseline to the 12‐month follow‐up were observed; however, social functioning levels remained stable thereafter. Among participants, 9% demonstrated consistently high/normal social functioning, 47% demonstrated improved social functioning, 13% were found to have worsening social functioning, and 32% demonstrated consistently low social functioning. AYA patients with cancer who had consistently low social functioning were more often off treatment at the time of follow‐up, reported more physical symptoms and higher levels of distress at baseline and follow‐up, and perceived less social support at baseline compared with the other 3 groups. CONCLUSIONS Although improved over time, social functioning still was found to be compromised 24 months after the primary diagnosis. Nearly one‐third of these patients remain at risk of poor social functioning. Reducing physical symptoms and psychological distress and enhancing social support by interventions during the period after treatment may potentially help these young survivors to better reintegrate into society. Cancer 2017;123:2743‐51. © 2017 American Cancer Society. Nearly one‐third of adolescent and young adult patients with cancer remain at risk of poor social functioning 24 months after their primary diagnosis. Reducing physical symptoms and psychological distress and enhancing social support by intervention during the period after treatment may potentially help these young survivors to better reintegrate into society.