The practice of binge-watching (i.e., watching multiple episodes of TV series in one session) has become increasingly prevalent, but comprehending its nature and potential underlying factors has been challenging. In particular, problematic binge-watching remains ill-defined and conceptualized, being regarded either as an addictive behaviour or a maladaptive emotion regulation strategy. Following a process-based approach, in the current study we explored the latter conceptualization, by investigating the potential mediating role of an unconstructive ruminative thinking style between negative affect and problematic binge-watching. To this end, TV series viewers completed an online survey assessing socio-demographic variables, TV series viewing habits, binge-watching motives and engagement, ruminative thinking styles and affect. Based on their answers, participants were allocated to one of the following three groups: non-binge-watchers (n = 59), trouble-free binge-watchers (n = 85), or problematic binge-watchers (n = 162). Group comparisons and mediation analyses were conducted to explore the underlying role of unconstructive rumination in problematic binge-watching. Results showed that, apart from the pattern of TV series watching, trouble-free binge-watchers shared little to no similarity with problematic binge-watchers, therefore supporting the need to differentiate these two behaviours. Moreover, mediation analyses revealed that an unconstructive ruminative thinking style partially mediated the relationship between negative affect and coping/escapism and that it fully accounted for the relationship between negative affect and binge-watching derived positive emotions in problematic binge-watchers. These findings thus add to the notion that problematic binge-watching might serve as a way to bolster a maladaptive emotion regulation strategy, implying that unconstructive rumination acts as a mediating process in this context.