Summary Background Mobile technology may help to better understand the adherence to treatment. MASK ‐rhinitis (Mobile Airways Sentinel NetworK for allergic rhinitis) is a patient‐centred ICT system. A mobile phone app (the Allergy Diary) central to MASK is available in 22 countries. Objectives To assess the adherence to treatment in allergic rhinitis patients using the Allergy Diary App. Methods An observational cross‐sectional study was carried out on all users who filled in the Allergy Diary from 1 January 2016 to 1 August 2017. Secondary adherence was assessed by using the modified Medication Possession Ratio ( MPR ) and the Proportion of days covered ( PDC ) approach. Results A total of 12 143 users were registered. A total of 6 949 users reported at least one VAS data recording. Among them, 1 887 users reported ≥7 VAS data. About 1 195 subjects were included in the analysis of adherence. One hundred and thirty‐six (11.28%) users were adherent ( MPR ≥70% and PDC ≤1.25), 51 (4.23%) were partly adherent ( MPR ≥70% and PDC = 1.50) and 176 (14.60%) were switchers. On the other hand, 832 (69.05%) users were non‐adherent to medications ( MPR <70%). Of those, the largest group was non‐adherent to medications and the time interval was increased in 442 (36.68%) users. Conclusion and clinical relevance Adherence to treatment is low. The relative efficacy of continuous vs on‐demand treatment for allergic rhinitis symptoms is still a matter of debate. This study shows an approach for measuring retrospective adherence based on a mobile app. This also represents a novel approach for analysing medication‐taking behaviour in a real‐world setting.