Background While there has been a clear move towards shared decision‐making (SDM) in the last few years, the measurement of SDM‐related constructs remains challenging. There has been a call for further psychometric testing of known scales, especially regarding validity aspects. Objective To test convergent validity of the nine‐item Shared Decision‐Making Questionnaire (SDM‐Q‐9) by comparing it to the OPTION Scale. Design Cross‐sectional study. Setting and participants Data were collected in outpatient care practices. Patients suffering from chronic diseases and facing a medical decision were included in the study. Methods Consultations were evaluated using the OPTION Scale. Patients completed the SDM‐Q‐9 after the consultation. First, the internal consistency of both scales and the inter‐rater reliability of the OPTION Scale were calculated. To analyse the convergent validity of the SDM‐Q‐9, correlation between the patient (SDM‐Q‐9) and expert ratings (OPTION Scale) was calculated. Results A total of 21 physicians provided analysable data of consultations with 63 patients. Analyses revealed good internal consistency of the SDM‐Q‐9 and limited internal consistency of the OPTION Scale. Inter‐rater reliability of the latter was less than optimal. Association between the total scores of both instruments was weak with a Spearman correlation of r = 0.19 and did not reach statistical significance. Discussion By the use of the OPTION Scale convergent validity of the SDM‐Q‐9 could not be established. Several possible explanations for this result are discussed. Conclusion This study shows that the measurement of SDM remains challenging.