How do heretical social movements build and negotiate their collective identities? This chapter tackles this question by examining the case of an emerging social movement, the left-wing Islamists in contemporary Turkey, that cuts across the durable divide between Turkey’s left and Islam. Drawing on four months of fieldwork in Turkey, I argue that, in addition to activating the typical “us versus them” dynamic of contentious politics, the left-wing Islamists also rely on blurring the social and symbolic boundaries that govern political divides in the course of building their collective identities. Their social boundary blurring includes facilitating otherwise unlikely face-to-face conversations and mutual ties between leftists and Islamists and spearheading alliances on common grounds including anti-imperialism and labor. Their symbolic boundary blurring includes performing a synthesis of Islamist and leftist repertoires of contention and reframing Islamic discourse with a strong emphasis on social justice and oppositional fervor. The case of Turkey’s left-wing Islamists illuminates the process of boundary blurring as a key dimension of collective identity and alliance formation across divides.