Originally an inlet of the sea ran northwards to a cove at present-day Hibiya and Kokyo Gaien plaza, into which the Kanda River flowed.

[2] On September 5, 1905, a protest at the park against the Treaty of Portsmouth, which ended the Russo-Japanese War (1904−1905), erupted into the Hibiya riots.

The major citywide riots lasted two days, with seventeen people being killed and 331 arrested, as well a large amount of property damage.

The riots were against the terms of the treaty, which were lenient to Russia, but also against bureaucrats who refused to accept the will of the people on foreign policy.

[3] The park is famous for the Shisei Kaikan (市政会館), a brick building built in Gothic style in 1929, which once housed the Domei Tsushin state wire service and its postwar successors Kyodo News and Jiji Press.