Background: More than half of patients with bipolar disorder (BD) experience anxiety, which is associated with impaired functioning. In patients with BD, the present study aimed (1) to validate daily patient-reported symptoms of anxiety measured using smartphones against clinically rated symptoms of anxiety, (2) to estimate the prevalence of anxiety symptoms, and (3) to investigate the associations between patient-reported anxiety symptoms and stress, quality of life and functioning. Methods: A total of 84 patients with BD evaluated their anxiety symptoms daily for nine months using a smartphone-based system. Data on clinically evaluated symptoms of anxiety and functioning and patient-reported stress and quality of life were collected from each patient at five fixed time points during follow-up. Results: The patients presented mild affective symptoms only. The reporting of anxiety symptoms was evaluated for validity according to clinically evaluated anxiety scores based on the two anxiety sub-items of the Hamilton Depression Rating Scale. The patients experienced symptoms of anxiety 19.3% of the time. There were statistically significant associations between anxiety and stress, quality of life and functioning (all p-values < 0.0001). Conclusion: In patients with BD in full or partial remission, the self-reporting of anxiety symptoms using smartphones was validated. Anxiety is associated with increased stress, decreased quality of life and functioning even during full or partial remission. Identifying anxiety symptoms thus has clinical impact, which suggests that smartphones may serve as a valid tool.