Hans Hass (23 January 1919 – 16 June 2013) was an Austrian biologist and underwater diving pioneer. He was known mainly for being among the first scientists to popularise coral reefs, stingrays, octopuses and sharks. He pioneered the making of documentaries filmed underwater and led the development of a type of rebreather. He is also known for his energon theory and his commitment to protecting the environment.

Hass was born in Vienna; his father was an attorney and Hass initially pursued law. However, Hass had a formative encounter with the American diver Guy Gilpatric while on a Riviera holiday in 1938 which included underwater hunting and photography. After making expeditions to the Caribbean Sea and writing his first professional articles in 1938-39, in 1940 Hass switched from reading law to studying zoology and graduated with a Ph.D. from the University of Berlin in 1943 at the Faculty of Biology. His thesis was the first scientific research project that used an autonomous rebreather diving equipment. In his early diving he used rebreathers, which he had made for him by the German diving gear makers Dräger: he had these sets made with the breathing bag on his back, as he did not like the bag-on-chest "frogman look". Hass and his team of researchers logged over 2000 dives utilising oxygen rebreathers from 1942 to 1953.

Although Don Stewart, one of the first scuba operators on the Caribbean island of Bonaire, blames Hass for single-handedly hunting the Atlantic goliath grouper to local extinction in a book by Callum Roberts, the author clearly refutes that claim later in the same paragraph.

Hass published "Diving to Adventure," his first book of underwater photographs, in 1939 and some credit him with developing one of the first underwater cameras. Hass completed his first underwater film called Pirsch unter Wasser (Stalking under Water) in 1940. It was published by the Universum Film AG, originally lasting only 16 minutes and was shown in cinemas before the main film, but would eventually be extended by additional filming done in the Adriatic Sea close to Dubrovnik.

Hass moved from Vienna to Berlin in 1941, where he founded the tax privileged society Expedition für biologische Meereskunde (Expedition for biological oceanography).

Hass was excused from serving in the German military during the Second World War because of poor circulation in his feet caused by Raynaud's disease.

From the proceeds of his hundreds of lectures, Hass was able to buy the sailing ship Seeteufel in 1942. However, he was not able to use the ship for his planned expedition because the ship was in the harbour of Stettin and it was not possible to bring it to the Mediterranean Sea during the war.

Therefore, Hass rented a ship in Piraeus and sailed for several months in the Aegean Sea and the Sea of Crete in 1942. Before the war, this ship had been owned by the University of Vienna. During this expedition he filmed and took photos underwater. Hass had read the book Die Raubfischer in Hellas (The Pirate Fishers in Greece) written in 1939 by Werner Helwig. Hass found this group near Skiathos and was able to film their dynamite fishing under water.