Sekizaki Ocean and Astronomical Observatory Hall, Observatory and science museum in Saganoseki, Oita, Japan.

This facility combines astronomical observation capabilities with educational exhibits about marine phenomena and celestial bodies, featuring telescopes and interactive displays that allow visitors to explore both oceanic and space science through hands-on demonstrations and guided observations.

The facility opened on 1 April 1995 as a lifelong-learning centre of the former Saganoseki town, under the official name Toyo Strait Sekizaki Kaiseikan. When Saganoseki merged into Oita city in 2005, the city's board of education took over and the name became Sekizaki Kaiseikan. It closed for renovation on 15 May 2022; JX Metal, which runs a smelter in Saganoseki, acquired the naming rights, and the hall reopened on 21 July 2023, with work completed that November.

The hall's name comes from the Japanese word for starfish, kai-sei, and it runs a stamp rally and talks by Japan Coast Guard officers on the annual open day of the nearby Sekizaki Lighthouse. In summer, workshops build model rockets, since launches from Tanegashima and Uchinoura can sometimes be seen from the site. Three of its directors have had asteroids named after them by the International Astronomical Union.

The entrance moved from the north to the south side in the 2023 renovation, and a new viewing area was added outside. Two accessible parking spaces were added, and a lift and platform give wheelchair users direct access to the paid observation room. The digital planetarium seats 22 and requires advance booking on the website; walk-ins are accepted if seats remain.

The building's exterior resembles the decagonal mansion in Yukito Ayatsuji's 1987 debut novel The Decagon House Murders, but the hall opened in 1995, eight years later, so it was not the model. The novel's Cape J offshore island is said to be based on Takashima, visible from the cape below.