Background: Most people with common mental health problems do not seek evidence-based psychological interventions. Aims: The aim of this study was to investigate whether monitoring symptoms of depression and anxiety using an app increased treatment-seeking. Method: Three hundred and six people with significant levels of anxiety and depression, none of whom were currently receiving treatment, were randomly allocated to receive either (a) information about local psychological services only, (b) information plus regular symptom monitoring (every 6 days), or (c) information plus open symptom monitoring (monitoring when they felt like it). An app was used to provide information and monitor mood. Results: The proportion of participants who reported receiving treatment after starting the study was 7.2% (10/138) in the information only group, 8.1% (9/111) in the information plus regular monitoring group and 15.8% (9/57) in the information plus open monitoring group. There was a trend for participants who were able to monitor whenever they wished to be more likely to report receiving treatment than people who were only given information about their local treatment services. The impact of the intervention was greatest among participants who intended to seek treatment before taking part. Limitations were that only a small minority of those who downloaded the app completed the study and that the study relied on self-reported measures of treatment-seeking. Conclusions: Symptom monitoring can increase actual treatment-seeking in those with an intention to seek treatment.