The tourist office of Thessaloniki opens late and closes early. It is a pretty big building, which is also the tourism ministry office, and is responsible for promoting the region. There is much information about the city, its Byzantine churches that are part of UNESCO world heritage, but also on the ancient ruins that surround the city, the islands of the archipelagos around and also how to reach the capital. What I think is a shame is that they assume that you're just going to visit the city, and are terrible for giving directions on how to get to the next town or futher, for example there was nothing about Albania or Macedonia, and the daily buses that go up there. Besides that, they were friendly, and spoke good English. They have maps of the city with the Byzantine churches marked.
It opens late and closes early. Located just behind the center. It is a large airy building, which is also the Ministry of Tourism, and manages the promotion of the region. It has a lot of information about Thessaloniki, its Byzantine churches included in UNESCO World Heritage Site, and the ancient sites around the city, the neighboring islands of the archipelagos and connections to the capital. We did not receive information on how to get to Macedonia or Albania, though we remained connected for hours. Yet good service, English speakers and they have good maps which feature the location of the churches.