Perseverative thought (also known as repetitive thought) is an established transdiagnostic mechanism of internalizing psychopathology characterized primarily by its repetitive, difficult to control quality. Subjective difficulty concentrating frequently coincides with perseverative thought and may function as both mechanism and clinical consequence of perseverative thought. Although cognitive theories suggest a bidirectional relationship between perseverative thought and difficulty concentrating, experimental and longitudinal investigations provide mixed evidence. The current study addresses this gap by testing a bidirectional relationship between momentary perseverative thought and difficulty concentrating using ecological momentary assessment (EMA) (8 surveys per day for 14 days) with 200 adult community participants (N = 155 with compliance ≥ 70 %) oversampled for moderate/severe perseverative thought. Multilevel modeling tested concurrent and time-lagged associations. Difficulty concentrating and perseverative thought were concurrently and prospectively related within-person: higher difficulty concentrating at T-1 predicted higher perseverative thought at T, and higher perseverative thought at T-1 predicted higher difficulty concentrating at T (β =.19 -.20, p <
.001). Findings held when controlling for the outcome at T-1 (β =.02 - .06, p <
.001). These findings demonstrate covariation between difficulty concentrating and perseverative thought within-person in daily life consistent with theoretical models that predict bidirectional relationships. Findings suggest that intervening on difficulty concentrating and/or perseverative thought may reduce the "vicious cycle" of perseverative thought episodes.