Artemios (Demis) Ventouris Roussos (Greek: Ντέμης Ρούσος, born June 15, 1946) is a Greek singer, best known for being the main musical partner of movie soundtrack composer and electronic musician Vangelis throughout the 1960s and 1970s. Roussos has sold over 40 million albums worldwide.

He was born in Egypt from Greek origin parents George (Engineer Yorgos Roussos) and Olga, and raised in Alexandria. His parents lost everything and moved back to Greece after the Suez Crisis.

After settling in Greece, Roussos participated in a series of musical groups beginning with The Idols when he was 17, when he met Evanghelios Papathanassiou (later known as Vangelis), and Loukas Sideras, his future bandmates in Aphrodite's Child from 1967 to 1972. After this he joined We Five (not the San Francisco, California folk-rock group), another covers band which had limited success in Greece.

He came to a wider audience in 1968 when he joined progressive rock band Aphrodite's Child, with Vangelis and Sideras, initially as singer but later also playing bass guitar, and achieving massive commercial success in France and Europe in 1968 until 1972. His distinctive operatic vocal style helped propel the band to international notoriety, notably on their final album 666, which became a progressive rock cult classic.