Ernst Julius Öpik (October 23, 1893 – September 10, 1985) was a notable Estonian astronomer and astrophysicist, who spent the last part of his career (1948–1981) at the Armagh Observatory in Northern Ireland.

Öpik specialized in the study of minor bodies, such as asteroids, comets, and meteors.

In 1922 he correctly predicted the frequency of craters on Mars long before they were detected by space probes.

In 1932 he postulated a theory concerning the origins of comets in our solar system. He believed that they originated in a cloud orbiting far beyond the orbit of Pluto. This cloud is now known to us as the Oort Cloud.

He won the Gold Medal of the Royal Astronomical Society in 1975 and the Bruce Medal in 1976.

The asteroid 2099 Öpik is named in his honour.

His grandson is Lembit Öpik, a politician in the United Kingdom.