St. Demetrius of Thessaloniki (Άγιος Δημήτριος της Θεσσαλονίκης in Greek) was a Christian martyr who lived in the 4th century. He is also referred to as "the Megalomartyr" by the Greeks.

Demetrius was a deacon in Thessaloniki. Early legends about Demetrius credit him with a military career in the Roman army, so he is also sometimes called a military martyr. He was run through with spears ca 306 at Sirmium, modern Serbia, during the Christian persecutions of emperor Maximian.

He was extremely popular in the Middle Ages, and along with St. George, he was the patron of the Crusades.

St. Demetrius is also commonly spelled as St. Dimitrios by Greek migrants in Australia and the United States, and commenly depicted in icons with the White Tower of Thessaloniki in the background. The tower depicted in the icons was build in the 16th century, centuries after his life, as the architecture of the older tower in the exact site is unknown. Szent Demeter Demetrius van Sirmium