Recent theoretical and empirical evidence highlights associations between attention-deficit/hyperactivity disorder (ADHD) symptoms and task-unrelated thought, including mind-wandering and rumination. However, it has been hypothesized that sluggish cognitive tempo (SCT), characterized by daydreaming and staring behaviors, may uniquely relate to task-unrelated thought. The purpose of the present study was to test whether SCT symptoms are associated with greater mind-wandering and rumination, and whether this association remains when controlling for ADHD and internalizing symptoms. Participants (N = 4679; 18-29 years; 69% female; 80.9% White) enrolled in six universities in the United States completed measures of SCT, ADHD symptoms, internalizing symptoms, and rumination, as well as two scales used to assess mind-wandering. Although ADHD symptoms were correlated with greater self-reported mind-wandering and rumination, relations with mind-wandering on the daydreaming frequency scale, reflective rumination, and brooding rumination were attenuated when controlling for SCT and internalizing symptoms. Above and beyond other psychopathology dimensions, SCT symptoms were uniquely associated with greater self-reported mind-wandering and both reflective and brooding rumination. Additionally, SCT symptoms were more strongly associated than other psychopathology dimensions with the mind-wandering measure of daydreaming frequency. Results provide the first empirical support for unique and robust associations between SCT symptoms and task-unrelated thought, while suggesting that the link between ADHD and mind-wandering may be less robust than previously suggested.