Objectives: Adaptive coping strategies are associated with less psychological distress. However, there is no brief, specific, and validated instrument for assessing adaptive coping among seriously ill patients. Our objective was to examine the validity and patient-proxy agreement of a novel instrument, the Sickness Insight in Coping Questionnaire. Design: A cross-sectional design which included two related studies. Setting: A single university-affiliated Dutch hospital. Subjects: Hospitalized patients (study 1) and ICU-patients and proxies (study 2). Interventions: None. Measurements and Main Results: Study 1 (n = 103 hospitalized patients) addressed the Sickness Insight in Coping Questionnaire’s performance relative to questionnaires addressing similar content areas. Coping subscales of the BRIEF COPE, Illness Cognition Questionnaire, and Utrecht Coping List were used as comparator measures in testing the construct validity of the Sickness Insight in Coping Questionnaire-subscales (fighting spirit, toughness, redefinition, positivism, and non-acceptance). The Sickness Insight in Coping Questionnaire had good internal consistency (0.64 ≤ α ≤ 0.79), a clear initial factor structure, and fair convergent (0.24 ≤ r ≤ 0.50) and divergent (r, ≤ 0.12) construct validity. Study 2 examined the performance of the Sickness Insight in Coping Questionnaire among 100 ICU patients and their close family members. This study showed that the Sickness Insight in Coping Questionnaire has good structural validity (confirmatory factor analyses with Comparative Fit Index > 0.90 and Root Mean Square Error of Approximation < 0.08) and moderate (r, 0.37; non-acceptance) to strong (r, > 0.50; fighting spirit, toughness, redefinition, and positivism) patient-close proxy agreement. Conclusions: Overall, the Sickness Insight in Coping Questionnaire has good psychometric properties. ICU clinicians can use the Sickness Insight in Coping Questionnaire to gain insight in adaptive coping style of patients through ratings of patients or their close family members.