You can reach the city by air, sea, rail, or road !
Sea means a cruise ship, trains are currently for domestic travel only, so if you are coming from abroad you can basically arrive by plane or car. With the prices of gas rising continuously, plane is now the cheapest form or travel. (There are also people who can arrive by bicycle but these are rare cases !)
Thessaloniki currently enjoys direct flight connections with the following cities.
Austria : Vienna
Belgium : Brussels
Czech Republic : Prague
France : Paris
Germany : All the main cities
Hungary : Budapest
Israel : Tel Aviv
Italy : Rome, Milan, Bergamo, Bologna
Norway : Oslo
Romania : Bucharest
Russia : Moscow, St. Petersburg
Spain : Barcelona ( Girona )
Switzerland : Bassel
Sweden : Stockholm
Serbia : Belgrade
Turkey : Istanbul
Ukraine : Kiev
UK : London, Manchester
These are the direct flights to Thessaloniki as they currently appear on skyscanner.com. Airline schedules change often, so check with your favourite travel website for latest schedules and best price offers.