Manuel Monteiro de Castro (born 29 March 1938) is a Portuguese prelate of the Catholic Church who worked in the diplomatic service of the Holy See from 1967 to 2009, with the rank of archbishop and the title of nuncio from 1985.

He then studied at the Pontifical Gregorian University in Rome, earning a doctorate in canon law in June 1967.

He served as secretary of the nunciatures in Panama from 1967 to 1969, Guatemala from 1969 until 1972, Việt Nam and Cambodia from 1972 until 1975, Australia from 1975 to 1978, and Mexico from 1978 to June 1981.

On 16 February 1985, Pope John Paul II appointed him titular archbishop of Beneventum; Apostolic Pro-Nuncio to the Bahamas, Barbados, Belize, Dominica, Grenada, Jamaica, Saint Lucia, and Trinidad and Tobago; and Apostolic Delegate to the other territories in the Antilles.

[13] He met with José Luis Rodríguez Zapatero to reduce tension between the Spanish government and the Church.

Addressing a conference of Spanish bishops in 2004, he proposed they support the legal recognition of same-sex relationships called something other than "marriages".

[19] On 6 January 2012, Pope Benedict XVI announced he would make Monteiro de Castro a cardinal on 18 February.