Theodorus of Samos (Greek: Θεόδωρος ὁ Σάμιος) was a sculptor and architect from the Greek island of Samos in the 6th century BC. He worked with another artist named Rhoecus and was often credited with developing the process of melting metal ores and casting metal. According to the ancient writer Pausanias, Theodorus was also responsible for creating the craft of casting. He is known for inventing tools such as the water level, the carpenter's square, and, as recorded by Pliny, the lock and key and the turning lathe. Vitruvius, an ancient Roman architect, stated that Theodorus designed the Doric order temple called the Heraion of Samos. Some historical texts describe him as a skilled artist, and in some statues, he is shown as an important inventor.

The ancient historian Herodotus referred to Theodorus as "the son of Telecles," a Samian artist. Herodotus noted that Theodorus and Rhoecus improved the method of mixing copper and tin to make bronze, and they were the first to use bronze in casting. In another account, Herodotus credited Theodorus alone with discovering how to melt iron and use it to cast statues.

In the television series Cosmos, the scientist Carl Sagan mentioned that Theodorus is credited with inventing the level, ruler, key, square, lathe, and the technique of bronze casting.

Theodorus wrote a book about the Third Temple of Hera at Samos. This book is the earliest known written work about architecture for which the author's name is recorded.