The Libro de Alexandre is a medieval Spanish epic poem about Alexander the Great written between 1178 and c. 1250 in the mester de clerecía.^[1] It is largely based on the Alexandreis of Walter of Châtillon, but also contains many fantastical elements common to the Alexander romance. It consists of 2,675 stanzas of cuaderna vía and 10,700 lines.

The Libro is preserved in two manuscripts, called P and O, neither of which appears to be an original.^[1] There are as well three fragments preserved in separate manuscripts. Manuscript O is the earlier, copied around 1300, and includes 2,510 stanzas of cuaderna vía and two epistles. It was once owned by the Duke of Osuna (whence O) and was known to Íñigo López de Mendoza, 1st Marquess of Santillana.^[1] P, from Paris, was copied in the fifteenth century and contains 2,639 stanzas. It is generally more reliable and together the two manuscripts make a coherent whole. R. S. Willis, Jr., produced an edition of both manuscripts where a page from O faces the corresponding page from P, with fragments noted at the bottom, so that one reading can readily be corrected by the other text.^[1] O is generally considered to be from eastern Castile, while P was copied in western Castile. The fragment G′ is named after Gutierre Díez de Gamés, who included stanzas from the first part of the Libro in his early fifteenth-century Victorial.^[2]

The date of composition is uncertain. However, it must postdate 1178, the earliest year when Walter completed the Alexandreis, and predate 1250, the approximate date of the Poema de Fernán González, which it influences. Some scholars have fixed the date as between 1202 and 1207.^[3] Besides the Alexandreis, the author of the Libro claimed many sources. In his own words: el uno que leyemos, el otro que oyemos / de las mayores cosas Recabdo vos daremos ("the one that we read, the other that we hear / of the greatest things collected we give you").^[1] These sources include the Historia de proeliis of Leo of Naples and several ancient authorities, including Leo's source, Quintus Curtius, Flavius Josephus, and the Pindarus Thebanus.^[1] The work of Isidore of Seville and the Old French Roman d'Alexandre were also consulted.

Structurally the Libro is a chronological story of Alexander's life set between an introduction in six stanzas and a conclusion in seven. There are digressions and authorial displays of erudition, but the narrative, from birth to death, is logical and smooth.^[1] The problem of authorship is unresolved. It has been variously attributed to Juan Lorenzo de Astorga (sometimes thought to be merely a scribe), Alfonso X of Castile, and Gonzalo de Berceo.^[2]

Following is a sample text from the Libro, with translations in Modern Spanish and English. This fragment^[4] sums up the fall of Alexander because of his pride.