Transitional justice (TJ) scholarship and practice often pins much hope on civil society. It generally assumes that civil society organisations demonstrate 'civility', have broad-based memberships, support liberal democratic values and promote TJ approaches based on liberal-legal justice strategies. Yet there is nothing inherently virtuous about civil society and in conflict-affected societies it often lacks these desired properties; it can be underdeveloped, unruly and disruptive. So, what role do, and should, 'uncivil' society groups play in TJ processes? To answer this question this article uses comparative case studies of the role of uncivil society groups in Bougainville and Timor-Leste. These cases exhibit similar broad cultural, socio-political and socioeconomic characteristics. In both cases uncivil society groups are organised around societal divisions, attempt to operate as alternatives to the state, oppose liberal democracy and liberal-legal TJ processes and engage in unruly behaviour, including violence and criminality. These uncivil society groups are not regarded as internationally legitimate, but they nevertheless have strong local legitimacy, particularly when the state is absent or weak. Consequently, this article concludes that in conflict-affected societies it is necessary to engage with groups regarded as both civil and uncivil to promote locally legitimate and effective TJ, and peace more broadly.