Béla IV (1206–May 3, 1270) was the king of Hungary between 1235 and 1270 and member of Arpad dynasty.

Béla was the son of King András (Andrew) II and Gertrude of Meran. In 1213 his mother was murdered by Hungarian magnates. His father failed to avenge Queen Gertrude's murder so it was left to Béla to track down and punish them, a campaign he finally completed some thirty years after her death.

In 1218 Béla was married to Maria Laskarina, a daughter of Emperor Theodore I Lascaris of Nicaea and Anna Angelina. They had two sons and seven daughters, of whom the most notable were:
1. Kinga, married to King Boleslaus V of Poland; she was canonized by the Roman Catholic Church in 1999
2. Stephen, King István V
3. Erzsébet, married to Duke Henry XIII of Lower Bavaria
4. Margit, canonized by the Roman Catholic Church in 1943 as St. Margaret. She lent her name to Margaret Island.
Béla's reputation as monarch, compared to that of his father, is generally perceived to have been good. He was a good administrator and on his accession, sought to counter corruption and to recover lost territory which had been given over to the magnates by his father.

Mongol Invasions
In 1238, Hungary was invaded by Cuman tribes fleeing the advancing Mongol hordes. Béla sought an alliance with the Cumans, and so he granted them asylum and betrothed his son and heir, Stephen, to the daughter of a Cuman khan named Kuthen. The Cumans (originally a pagan shamanist people) converted to Christianity and were baptised. They fought beside the Hungarians against the Mongols.

Béla tried with little success to reestablish royal preeminence by reacquiring lost crown lands. His efforts, however, created a deep rift between the crown and the magnates just as the Mongols were sweeping westward across Russia toward Europe. Aware of the danger, Béla ordered the magnates and lesser nobles to mobilize. Few responded. Béla also sent messages to Pope Gregory IX and the Holy Roman Emperor Frederick II but to no avail. The Mongols eventually routed Béla's army at the Battle of Mohi on April 11, 1241. His ally Kuthen had been killed by mistrustful Hungarian lords in Pest just prior to the invasion.

Béla fled to Austria, where Duke Frederick of Babenberg held him for ransom, then to Trogir in Dalmatia. The Mongols reduced Hungary's towns and villages to ashes and slaughtered half the population before news arrived in 1242 that the Great Ögedei Khan had died in Karakorum. The Mongols withdrew, sparing Béla and what remained of his kingdom.

Upon his return to power, Béla began rebuilding his country, including a massive construction campaign which produced the system of castles as a defence against the threat of a Mongol return. This eventually happened in 1261 but this time Béla was successful in defeating them. He is greatly respected in Hungary and commonly known as "the second founder" of the kingdom.

Other Wars
Béla was determined to regain the western part of Hungary which had been seized by Frederick II as his price for giving Béla assistance in the first war against the Mongols (help which never came). Béla finally defeated Frederick in battle in 1246, Frederick being trampled to death by his own cavalry. Béla also engaged in a long battle with Otakar II of Bohemia for control of Austria and Styria but this was unsuccessful. He was regularly engaged in protecting the outer extremeties of his realm including Dalmatia, Bosnia and Serbia.

The final years of Béla's reign were marred by the rebellion of his son Stephen. Béla was eventually forced to divide his kingdom in two, with Stephen setting up his own capital and adopting foreign policies directly contrary to those of his father.