Penzias, Arno Allan (1933-) is a German-born American astrophysicist who discovered and studied cosmic microwave background radiation. In 1978, he won the Nobel Prize in physics, sharing it with his American colleague Robert Woodrow Wilson.

Penzias was born on April 26, 1933, in Munich, Germany, one of two sons of Karl Penzias, who ran a small leather business, and Justine Eisen-reich Penzias. The Jewish family fled Nazi Germany, arriving in New York City in 1940. Penzias studied at Columbia University in New York City, earning a doctorate in 1962. He then joined Bell Telephone Laboratories (which became part of Lucent Technologies) in Holmdel, New Jersey.

In 1963, Bell assigned Penzias and Wilson to trace radio noise that was interfering with earth to satellite communications. While exploring radio waves in space, Penzias and Wilson noticed a uniform background static suggesting radio waves with a residue of heat energy of a certain temperature. This faint warmth is now generally held to be the result of the remaining background radiation resulting from the explosion in which the universe is thought to have been created. This explosion is known as the big bang, a hot, explosive beginning of the universe 10 billion to 20 billion years ago. The radio waves are similar to waves that would have been received from an extremely hot object moving rapidly away from the earth. The discovery of the radio waves reinforced the idea that the universe is expanding.

Penzias and Wilson shared half of the 1978 Nobel Prize in physics for their discovery of cosmic microwave background radiation. The other half of the prize was awarded to Pyotr Kapitsa.

Penzias was a member of the National Academy of Sciences, the American Academy of Arts and Sciences, and the American Astronomical Society. His other honors include the Henry Draper Medal of the National Academy of Sciences (1977), the Herschel Medal of the Royal Astronomical Society in London (1977), and an honorary degree from the Paris Observatory.

In 1981, Penzias became vice president of research at Bell and retired from Lucent in 1998.