Pytheas of Massalia (/ˈpɪθiəs/; Ancient Greek: Πυθέας ὁ Μασσαλιώτης Pythéās ho Massaliōtēs; Latin: Pytheas Massiliensis; born c. 350 BC, fl. c. 320–306 BC) was a Greek geographer, explorer and astronomer from the Greek colony of Massalia (modern-day Marseille, France). He made a voyage of exploration to Northern Europe in about 325 BC, but his account of it, known widely in antiquity, has not survived and is now known only through the writings of others.
On this voyage, he circumnavigated and visited a considerable part of the British Isles. He was the first known Greek scientific visitor to see and describe the Arctic, polar ice, and the Celtic and Germanic tribes. He is also the first person on record to describe the midnight sun. The theoretical existence of some Northern phenomena that he described, such as a frigid zone, and temperate zones where the nights are very short in summer and the sun does not set at the summer solstice, was already known. Similarly, reports of a country of perpetual snow and darkness (the country of the Hyperboreans) had reached the Mediterranean some centuries before.