Andrew of Longjumeau (also Longumeau, Lonjumel, etc.) was a 13th century French Dominican, explorer and diplomat.

He accompanied the mission under Friar Ascehn , sent by Pope Innocent IV to the Mongols in 1247; at the Tatars camp near Kars he met a certain David, who next year (1248) appeared at the court of King Louis IX of France in Cyprus. Andrew, who was now with Saint Louis, interpreted to the king David's message, a real or pretended offer of alliance from the Mongol general Ilchikdai (Ilchikadai), and a proposal of a joint attack upon the Islamic powers for the conquest of Syria.

In reply to this the French sovereign dispatched Andrew as his ambassador to the great Kuyuk Khan; with Longjumeau went his brother (a monk) and several others — John Goderiche, John of Carcassonne, Herbert "le sommelier," Gerbert of Sens, Robert a clerk, a certain William, and an unnamed clerk of Poissy.

The party set out about February 16, 1249, with letters from King Louis and the papal legate, and rich presents, including a chapel-tent, lined with scarlet cloth and embroidered with sacred pictures. From Cyprus they went to the port of Antioch in Syria, and thence travelled for a year to the khan's court, going ten leagues a day. Their route led them through Persia, along the southern and eastern shores of the Caspian (whose inland character, unconnected with the outer ocean, their journey helped to demonstrate), and probably through Talas, north-east of Tashkent.

On arrival at the supreme Mongol court — either that on the Imyl river (near Lake Ala-kul and the present Russo-Chinese frontier in the Altai), or more probably at or near Karakorum itself, south-west of Lake Baikal — Andrew found Kuyuk Khan dead, poisoned, as the envoy supposed, by Batu's agents. The regent-mother Ogul Gaimish (the "Camus" of Rubruquis) seems to have received and dismissed him with presents and a letter for Louis IX, the latter a fine specimen of Mongol insolence. But it is certain that before the friar had quitted "Tartary" Mangu Khan, Kuyuk's successor, had been elected.

Andrew's report to his sovereign, whom he rejoined in 1251 at Caesarea in Palestine, appears to have been a mixture of history and fable; the latter affects his narrative of the Mongols' rise to greatness, and the struggles of their leader, evidently Genghis Khan, with Prester John; it is still more evident in the position assigned to the Tatars homeland, close to the prison of Gog and Magog. On the other hand, the envoy's account of Tatar manners is fairly accurate, and his statements about Mongol Christianity and its prosperity, though perhaps exaggerated (e.g. as to the 800 chapels on wheels in the nomadic host), are based on fact.

Mounds of bones marked his road, witnesses of devastations which other historians record in detail; Christian prisoners, from Germany, he found in the heart of "Tartary" (at Talas); the ceremony of passing between two fires he was compelled to observe, as a bringer of gifts to a dead khan, gifts which were of course treated by the Mongols as evidence of submission. This insulting behaviour, and the language of the letter with which Andrew reappeared, marked the mission a failure: King Louis, says Joinville, "se repenti fort."

We only know of Andrew through references in other writers: see especially William of Rubruck's in Recueil de voyages, iv. (Paris, 1839), pp. 261, 265, 279, 296, 310, 353, 363, 370; Joinville, ed. Francisque Michel (1858, etc.), pp. 142, etc.; Jean Pierre Sarrasin, in same vol., pp. 254-235; William of Nangis in Recueil des historiens des Gaules, xx. 359-367; Rémusat, Mémoires sur les relations politiques des princes chrétiens … avec les … Mongols (1822, etc.), p. 52.