Objective: To develop a valid and reliable tool to measure triadic decision making between older adults with multiple chronic conditions (MCC), their informal caregivers and geriatricians. Methods: Video observational study with cross-sectional assessment of interaction during medical consultations between geriatricians (n = 10), patients (n = 108) and informal caregivers (68) by three calibrated raters at the geriatric outpatient department of two Dutch hospitals. The Observer OPTIONMCC instrument was developed, based on the 'Dynamic model of SDM in frail older patients' and the 'Observing Patient Involvement in Decision Making - 5 item scale' (Observer OPTION-5). Results: Factor analysis confirms that it is acceptable to regard the new scale as a single construct. The 7-item single factor solution explained 62.76% of the variability for geriatricians, 61.60% of the variability for patients and 54.32% of the variability for informal caregivers. The inter-rater ICC for the total Observer OPTIONMCC score was .96, .96, and .95 (resp. geriatricians, patients, informal caregivers), with values ranging from .60 to .95 for individual items, showing good levels of agreement. Conclusion and Practice Implications: We conclude that Observer OPTIONMCC is sufficiently valid and reliable to be used for the assessment of triadic SDM in populations of older patients with MCC.