Kenmotsudai Arboretum, Arboretum in Kumamoto Castle Park, Japan.

Kenmotsudai Arboretum is a botanical collection within Kumamoto Castle Park featuring many tree species and plants arranged along walking paths. The collection forms a distinct area inside the larger park with organized plant zones for different species.

The arboretum opened in November 1952 as a botanical research location in post-war times. It was created as a place for scientific study and plant education.

The name Kenmotsudai comes from Nagaoka Kenmotsu, a senior retainer of the Hosokawa family who guarded the north side of Kumamoto Castle. In fact the arboretum stands on land that belonged to Nagaoka Tosho, and the site was mistakenly named after Kenmotsu. Visitors can also see a 762 mm diesel locomotive from the Uchidaidaishi Forest Railway in the grounds.

The arboretum stands at 4-1 Ninomaru, Chūō-ku, near the Kenmotsu turret at the castle's north end. From Sakura-machi Bus Terminal take the Kumamoto Castle loop bus to the Kumamoto Castle stop, then walk about five minutes; from the Kumamoto Castle/City Hall tram stop it is about fifteen minutes on foot. It closes on Mondays except from 20 March to 31 May, and over the New Year holidays.

The garden's name is a mistake: it honours Nagaoka Kenmotsu, a senior retainer of the Hosokawa lords of Kumamoto Castle, but the grounds actually belonged to Nagaoka Zusho. Among the trees stands a 762 mm-gauge diesel locomotive from the Uchidaime Forest Railway in the former Yabe forestry district.