Marcin Kromer (Latin: Martinus Cromerus; 11 November 1512 – 23 March 1589) was Prince-Bishop of Warmia (Ermland), a Polish cartographer, diplomat and historian in the Kingdom of Poland and later in the Polish–Lithuanian Commonwealth.

Kromer was born in 1512 into a prominent burgher family of German descent in Biecz, in Lesser Poland.

In 1564 Kromer was recalled to Poland, where he was promoted within the church hierarchy and took the post of coadjutor (de facto bishop) of the Bishopric of Warmia, to succeed on the demise of Prince-Bishop Stanislaus Hosius.

One of his notable demands was providing the Cracow Academy with new privileges to restore its position as one of the renowned universities in Central Europe.

Martin Kromer and Stanislaus Hosius (Stanisław Hozjusz) were the two bishops most instrumental in causing Royal Prussia's diocese of Warmia to return to or remain Catholic during a time of major conversions to Protestantism, especially in the neighboring Duchy of Prussia which almost surrounded the Prince-Bishopric of Warmia, and had converted to Lutheranism as the first state to do so.