The app is available for Android and Apple mobile phones and uses bluetooth technology.
The success of the app is based on the conscientious and solidary participation of citizens, said Health Minister Vili Beros.
After the pandemic is over, it will no longer be possible to use the app, which is secure and uses temporary and anonymous data.
The app's cross-border interoperability has also been ensured.
The app was designed under a government decision by the APIS IT agency for support to information systems and technologies.
"All data exchanged between two devices, i.e. two persons, is recorded only on mobile devices," said Sasa Bilic, chairman of APIS IT's management board.
Every user is informed anonymously that they have been in contact with an infected persons some time over the past 14 days. Users will be notified by text if the contact has been high, medium or low risk based on precisely defined parameters.
"We took distance into account. A text about a high risk is when a person has been two metres away for more than 15 minutes over the past 48 hours," said epidemiologist Tomislav Benjak.
The user will receive a text saying they have been in high-risk contact, to stay home and contact their family doctor. Texts about medium and low risk contacts are created similarly.
A dozen EU member states (Austria, the Czech Republic, Italy, Latvia, Germany, Poland, Denmark, Ireland, France and Hungary) have designed mobile apps to track contacts and inform users about exposure to COVID-19.
Bozinovic: App is secure, has all the necessary certificates
Interior Minister Davor Bozinovic, who heads the national COVID-19 response team, said the Stop COVID-19 app had all the necessary certificates.
"The European Commission said the health authorities in each member state should manage the app. Simply put, it's communication between two mobile phones. The person who possibly receives a text that they have been near an infected person doesn't know who this person is, and the infected person can activate the app only if they want to," he told press.
No third party can know anything about that communication, he said. "It couldn't be more secure. After all, the app has all the necessary certificates."