The National Observatory (Portuguese: Observatório Nacional or ON)[1] is an institution localized in Rio de Janeiro, Brazil.

According to Serafim Leite [pt], the Jesuits installed an observatory in his school in Castle Hill in the Rio de Janeiro in 1730.

In the same place, in 1780, Portuguese astronomers Sanches d'Orta and Oliveira Barbosa set up an observatory and began to perform regular observations of astronomy, meteorology and terrestrial magnetism.

He was installed in the tower of the Military School, and was directed initially by professor of mathematics Pedro de Alcântara Bellegarde.

In 1845 the Minister of War, Francisco Jeronimo Coelho, reorganized the institution as Imperial Observatory of Rio de Janeiro, when he assumed the position of Director, Professor Soulier Sauve, Military School, who moved to the fortress of Conception, and, in 1846, had its first Regulation approved by decree.

Between 1846 and 1850, the Director of the Observatory Soulier moved again, this time to the former premises of a church in Castle Hill, where he remained until 1920 After the death of Soulier in 1850, Lieutenant Colonel Engineer Antonio Manoel de Mello, also a professor at the Military Academy, was named director, remaining in office until 1865, when it was replaced by Lieutenant Commander Anthony Joaquim Cruvelo d'Avila.

In February 1981, JA de Freitas Pacheco, director of ON, opened a site under the name Brazilian Astrophysical Observatory (OAB).