The aim was to evaluate patient self-assessment of RA disease activity in terms of Routine Assessment of Patient Index Data (RAPID) scores via a Web-based smartphone application (WebApp). In this prospective, multicentre study, adult RA patients were examined by a rheumatologist at baseline and after 3 months. Patients were asked to complete WebApp questionnaires weekly. The time course of patient-assessed RAPID3/4 scores and their correlations with rheumatologist-assessed DAS28, as well as Clinical and Simplified Disease Activity Indices (CDAI/SDAI), were evaluated. Eighty patients were included in the analysis (median RA duration, 4.5 years; age, 57 years; 59% female). At baseline, there was a moderate to strong correlation between RAPID3 and DAS28 (r = 0.63), CDAI (r = 0.65) and SDAI (r = 0.61) scores. Similar or stronger correlations were seen at the 3-month follow-up visit (DAS28 r = 0.66, CDAI r = 0.71 and SDAI r = 0.61). Similar correlations were seen between RAPID4 and rheumatologist assessments. Correlations were not influenced by demographics or RA treatment. In the 3-month period, the RAPID3 score changed into a higher severity category than the category at baseline at least once in 47% of patients. When DAS28 scores were predicted from the RAPID3, 11% of patients had an increase of > 1 DAS28 unit during the 3-month observation period. Web-based patient assessments were strongly correlated with rheumatologist assessments of RA activity and showed considerable variation during follow-up. This provides a rationale for further exploration of their use as cost-effective tools to monitor RA activity between outpatient visits and to optimize tight control strategies.