Arne Dekke Eide Næss (/ˈɑːrnə ˈnɛs/ AR-nə NESS; Urban East Norwegian: [ˈɑ̂ːɳə ˈdɛ̂kːə ˈæ̂ɪdə ˈnɛsː]; 27 January 1912 – 12 January 2009) was a Norwegian philosopher who coined the term "deep ecology", an important intellectual and inspirational figure within the environmental movement of the late twentieth century, and a prolific writer on many other philosophical issues. Næss cited Rachel Carson's 1962 book Silent Spring as being a key influence in his vision of deep ecology. Næss combined his ecological vision with Gandhian nonviolence and on several occasions participated in direct action.

Næss averred that while western environmental groups of the early post–World War II period had raised public awareness of the environmental issues of the time, they had largely failed to have insight into and address what he argued were the underlying cultural and philosophical background to these problems. Næss believed that the environmental crisis of the twentieth century had arisen due to certain unspoken philosophical presuppositions and attitudes within modern western developed societies which remained unacknowledged.

He thereby distinguished between what he called deep and shallow ecological thinking. In contrast to the prevailing utilitarian pragmatism of western businesses and governments, he advocated that a true understanding of nature would give rise to a point of view that appreciates the value of biological diversity, understanding that each living thing is dependent on the existence of other creatures in the complex web of interrelationships that is the natural world.

Næss was born in Slemdal, Oslo, Norway, the son of Christine (Dekke) and Ragnar Eide Næss. Ragnar was a successful banker, and Arne was the younger brother of shipowner Erling Dekke Næss. Næss had two children with his first wife Else and was the uncle of mountaineer and businessman Arne Næss Jr. (1937–2004).

In 1939, Næss was the youngest person to be appointed full professor at the University of Oslo and the only professor of philosophy in the country at the time.

He was a noted mountaineer, who in 1950 led the expedition that made the first ascent of Tirich Mir (7,708 metres or 25,289 feet) in Pakistan, depicted in the documentary film Tirich Mir til topps. The Tvergastein hut in the Hallingskarvet massif played an important role in the name of Næss's philosophy, "Ecosophy T", as "T" is said to represent his mountain hut Tvergastein.

In 1958, he founded the interdisciplinary journal of philosophy Inquiry.

In 1970, together with a large number of protesters, he chained himself to rocks in front of Mardalsfossen, a waterfall in a Norwegian fjord, and refused to descend until plans to build a dam were dropped. Though the demonstrators were carried away by police and the dam was eventually built, the demonstration launched a more activist phase of Norwegian environmentalism.