Purpose: Measuring exercise adherence is important in patients with chronic obstructive pulmonary disease (COPD). For this, the Rehabilitation Adherence Measure for Athletic Training (RAdMAT) seems to be a promising instrument, and a Dutch version (RAdMAT-NL) is available. The aim of this study was to explore the dimensionality and construct validity of the RAdMAT-NL in patients with COPD. Secondly, we examined whether the items of the RAdMAT-NL could be summed to a single score. Patients and Methods: This prospective study included 193 patients with COPD from 53 primary physiotherapy practices in The Netherlands and Belgium. Patients and their physiotherapist provided data including the RAdMAT-NL, at one, two, and three months after inclusion. Horn’s parallel analysis and exploratory factor analysis (EFA) were used to assess the dimensionality of the RAdMAT-NL. Fit to the dichotomous Rasch model for measurement was used to confirm the unidimensionality of the extracted RAdMAT-NL subscales and total scale. To evaluate construct validity, Spearman correlations with other indicators of adherence were calculated, including SIRAS score, percentage attendance and change in exercise skills. Results: EFA identified two dimensions of the RAdMAT-NL, “Participation” (13 items) and “Communication” (3 items), explaining 50.8% of the total variance. Rasch analysis confirmed the unidimensionality of the two dimensions. The unidimensional Rasch model was rejected for a summed score of all 16 RAdMAT-NL items. Medium to large significant positive correlations between the RAdMAT-NL subscale participation and different measures of adherence supported its convergent validity. Conclusion: The RAdMAT-NL exhibited two subscales that fitted the unidimensional Rasch model for objective measurement. Construct validity was supported by convergence with other established measures of adherence.