If you do not have a car: From passenger terminal D you can use bus line no. 2 to go to the Tallinn coach station. Bus line no 2 runs between the passenger terminal D (Bus stop: “Reisisadam D-terminal”)  and the airport via city centre.

Going to the Tallinn train station (Baltic Station/Balti jaam) requires a bit more walking and taking a tram or changing buses.

Please read also Getting there by bus or Getting there by train, respectively.

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Adventurous people with cars might also look for ferries meant for trucks and freight transportation. It is totally on your own risk… 😉