The origin of the name of the island is unknown. According to a view, the name derives from the small insect cricket (in Greek trizoni). In accordance with another view the name derives from corruption of name trionisia that means three islands (because the small group of islands comprises three islands).
Edward Blaquiere, a former lieutenant in the Royal Navy and a well known philhellene of his time, visited Trizonia in 1825 and he mentions a ruined village. He also mentions the excellent harbour of the island which is sheltered from all winds. The Greek military leader and politician Andreas Londos came to Trizonia in 1827 with 500 soldiers. In the ensuing battle with the 2000 Ottomans who were there at that time, the Greeks under the guidance of Londos won, causing great damage to the Ottoman army.
Trizonia is a settlement and an island, the largest island in the Corinthian Gulf with an area of 2.5 km2 and the only inhabited. They belong to the Prefecture of Fokida and together with Spilia and Chania they belong to the Local Community of Trizonia of the Municipal Unit of Tolofonos of the Municipality of Dorida (formerly in the Municipality of Tolofonos). The population of the island's settlement is 64 inhabitants, according to the 2011 census.
The island is part of a complex that includes three other smaller uninhabited islands, Agios Ioannis, Planemi and Prasoudi.
Its highest point, Aetorachi, is located at an altitude of about 106 meters above sea level. The vegetation of the island includes olives, almonds, vines, prickly pears, tree cypress, dense spruce, holly and eucalyptus. Due to its special aesthetic value, it has been characterized as a "Landscape of Special Natural Beauty".
The climate on the island of Trizonia is Mediterranean with relatively cool but humid summers and relatively mild winters.
The inhabitants of the island are engaged in fishing and tourism.