Background: School bullying is a group phenomenon in which being defended by peer bystanders may buffer against the development of psychological problems in victims. Aims: This registered report examines whether being defended diminished victims' daily mood problems and self-blame, both from a within- and between-person perspective. Materials and Methods: Daily diary data were collected from n = 1669 Finnish 7th-9th grade students (M age = 14.45; 55.5% girl) across 3 weeks. In n = 1329 out of 12,366 assessments (10.7%), students indicated that schoolmates victimized them on the day of bullying. Results: Multi-level regression analyses indicated that students reported lower depressed mood, greater positive mood and lower self-blame on days that they were victimized and defended as compared to days when they were victimized but non-defended. Effect sizes were medium for depressed mood and small for positive mood and self-blame. Repeated victims (n = 144) were less likely to blame themselves for victimization on days they were defended, which, in turn, diminished feelings of humiliation (mediation). Discussion: Our findings indicate that being defended benefits victims of bullying by mitigating mood problems, both directly and indirectly via diminished self-blame. Conclusion: Anti-bullying programmes that encourage peer defending have the potential to improve victims' psychological adjustment, even on a daily basis.