In paper degradation studies the viscosity-average degree of polymerisation (DP v) is often used as a key indicator of the extent of degradation of cellulosic paper. The DP v can be deduced from the viscosity of dilute paper solutions, as typically measured through glass capillary viscometry. The current study proposes an efficient, alternative method to evaluate the DP v of cellulosic paper, which is based on rotational rheometry. The proposed methodology relies on the application of a shear flow in a thin film of fluid to measure its dynamic viscosity, from which the DP v can be subsequently derived in a straightforward fashion. Rheometry allows to measure the viscosity for a range of shear rates, which results in multiple DP v evaluations per specimen, making the method statistically representative for each test with less experimental effort. The proposed viscometry methodology has been applied to 4 hygrothermally-aged cellulose paper samples and the unaged counterpart. The measurement results regarding the age dependency of the degree of polymerisation and the number of cellulose chain scissions are compared to those obtained by glass capillary viscometry, showing a very good agreement. Both experimental methods reveal a non-linear decrease in time of the degree of polymerisation, and a non-linear increase in time of the number of cellulose chain scissions, indicating that the cellulose ageing process is realistically captured. The agreement in measurement results further demonstrates that rheometry is an easy-to-use, accurate and efficient alternative for DP v measurements by glass capillary viscometry.