Perseverative thinking and catastrophizing have well established associations with fear and distress. However, less is known about the impact of interpersonal dynamics, such as co-rumination, on these intrapersonal cognitive processes and subsequent stress. The present study addresses this knowledge gap. A sample of 433 adults from across the United States was recruited online and completed measures of co-rumination, perseverative thinking, catastrophizing, and demographic characteristics early in the COVID-19 pandemic, and the COVID Stress Scales (CSS) at six month follow up. Co-rumination, perseverative thinking, catastrophizing, and CSS scores were correlated in the expected direction. Regression analyses revealed all three independently predicted CSS worry about the dangerousness of COVID-19 subscale. Co-rumination was the strongest predictor of CSS worry about the socioeconomic impact and CSS compulsive checking scales. Perseverative thinking and catastrophizing predicted CSS traumatic stress symptoms subscale. Finally, perseverative thinking was the strongest predictor of CSS xenophobia subscale. Structural equation modelling indicated that co-rumination had a significant indirect effect on CSS scores through perseverative thinking and catastrophizing. Interpersonal dynamics, such as co-rumination, are relevant for understanding stress and are promising targets for intervention research to prevent or attenuate fears and distress, in addition to traditional intrapersonal cognitive processes such as perseverative thinking and catastrophizing.