OBJECTIVE: In the Basque Country, two cases of COVID-19 were diagnosed on February 28 2020. On March 14, the Spanish Government established a state of alarm. Only cases confirmed by molecular biology (reverse-transcriptase polymerase chain reaction [RT-PCR]) were known. We launched a web-based surveillance tool to estimate the number of symptomatic cases of COVID-19 to contribute to Public Health decision-making. MATERIAL AND METHODS: We implemented an anonymous web questionnaire and disseminated it through online social media social. We collected epidemiological information about <<time>> (date of onset of symptoms), <<place>> (zip code), and <<person>> (gender, age). We compared cases detected by RT-PCR with the estimated cases, according to the case definition of the Ministry of Health. We calculated the questionnaire response rate and the cumulative incidence at 14days. RESULTS: Between March 19 and 26, 128,009 people answered the questionnaire (5.5% of the Basque population). Of these, 26,375 met the case definition (symptom prevalence of 21.4%). The estimated cases were almost six times more than COVID-19 positive RT-PCR. The estimated 14-day cumulative incidence was 578.3 per 100,000 population compared to RT-PCR positive cases, which was 139.6 per 100,000 population. CONCLUSIONS: This tool was useful in estimating the minimum number of symptomatic cases in the Basque Country, which could support Public Health actions.