Background: Patients who present to hand surgery practices are at increased risk of psychological distress, pain, and disability. Greater catastrophic thinking about pain is associated with greater pain intensity, and initial evidence suggest that, together, catastrophic thinking about pain and cognitive fusion (i.e., interpretation of thoughts as true) are associated with poorer pain outcomes. Purpose: We tested whether cognitive fusion or catastrophic thinking interacts in relation to pain and upper extremity physical function among patients seeking care from a hand surgeon. Methods: Patients (N = 110; mean age= 47.51; 59% women) presenting to an outpatient hand surgery practice completed computerized measures of sociodemographics, pain intensity, cognitive fusion, catastrophic thinking about pain, and upper extremity function. Results: ANCOVA revealed an interaction between cognitive fusion and catastrophic thinking about pain with respect to pain intensity and upper extremity function (ps < .01). Participants who scored high on both cognitive fusion and catastrophic thinking about pain reported the greatest levels of pain, relative to those who scored high on a single measure. The lowest levels of upper extremity function were also observed among those who scored high on both catastrophic thinking about pain and cognitive fusion. A similar pattern of results was observed when we tested each catastrophizing subscale individually. Conclusion: Maladaptive cognitions about pain (i.e., catastrophic thinking) may be particularly problematic when interpreted as representative of reality (i.e., cognitive fusion). Psychosocial interventions addressing catastrophic thinking about pain and cognitive fusion concurrently merit investigation among people with hand and upper extremity illness.