Objective/Background: According to the Cognitive Model of Insomnia disorder, rumination about lack of sleep and its diurnal consequences plays a crucial role in maintaining insomnia. Consolidated evidence shows that rumination is related to poor executive functions, which are cognitive control processes impacted by insomnia. Despite this evidence, no studies so far investigated the relationship between executive functions and rumination in individuals with insomnia. The aim of this pilot study was to cover this gap by investigating whether poor executive functions are associated with rumination in a sample of individuals with a diagnosis of insomnia disorder. Participants: Thirty young adults (22.67 ± 3.68 years, 73.3% females) diagnosed with insomnia disorder by clinical psychologists with expertise in behavioral sleep medicine completed the study. Methods: Measures of insomnia, depression, emotion regulation, and rumination about the daytime consequences of insomnia were collected. Executive functions were assessed using a Task Switching paradigm, measuring cognitive inhibition and set-shifting with cognitive flexibility. Results: Hierarchical multiple regression analysis revealed that higher depression (β = 0.781, p < 0.001) and cognitive reappraisal (β = 0.329, p = 0.016), and poorer cognitive inhibition (β = −0.334, p = 0.014), significantly predicted higher rumination. Conclusions: Rumination about symptoms of insomnia in a clinical sample is associated with impaired inhibitory but not switching capacities above and beyond the role played by traditional predictors such as depression and emotion regulation strategies. If replicated, present preliminary results suggest the need to target cognitive inhibition deficits in insomnia treatment.