Onzansō-en, Japanese strolling garden in Kainan, Japan.

Onzansō-en is a tidal pond strolling garden in Kainan, built as the villa garden of Nitta Chōjirō, founder of Nitta. Three ponds fed by seawater rise and fall with the tide around the main house, with tea rooms and tatami rooms scattered among them. A tunnel on the grounds opens onto a view of Kurae Bay.

The garden was created between the early Taishō and early Shōwa eras by Kizu Rinsai, head of the Mushanokōji Senke tea school, as the villa garden of Nitta Chōjirō, founder of Nitta. Its name was chosen by Admiral Tōgō Heihachirō from Nitta's pen name Onzan. The main house dates from 1915, and the garden is now a nationally designated Place of Scenic Beauty.

The garden's name was chosen by Admiral Tōgō Heihachirō, taking the pen name Onzan of the villa's owner, Nitta Chōjirō, the founder of Nitta. Plaques with calligraphy by Tōgō, Katsura Tarō, Kiyoura Keigo and Akiyama Saneyuki hang in the main building, the Onzansō, recalling the imperial family members and statesmen who visited this Kishū garden.

The garden is easily accessible through Kainan and features informational displays about native plant species throughout the grounds. Wear comfortable footwear since the terrain is uneven with multiple levels to explore.

The garden's name was chosen by Admiral Tōgō Heihachirō, who took it from Nitta Chōjirō's pen name, Onzan. Plaques inscribed by Tōgō, Katsura Tarō, Kiyoura Keigo and Akiyama Saneyuki, guests who accompanied visiting imperial family members, hang in the main house, Onzansō.