Béla IV (Croatian: Bela III.; 1206–May 3, 1270) was the king of Hungary and Croatia between 1235 and 1270 and member of Árpád dynasty.

Béla was the son of King András II and Gertrude of Merania. 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:

# Kunegunda, also known as Kinga, who was married to King Boleslaus V of Poland, after his death becoming a nun and abbess; she was canonized by Pope John Paul II in 1999 # Yolanda (Jolenta) married to the Grand Duke Boleslaus the Pious, and who also later became a nun and abbess, who has been declared as a candidate for sainthoood # Stephen, who succeeded him and ruled as Stephen V of Hungary # Elizabeth, married to Duke Henry XIII of Lower Bavaria # Constance, married to Knyaz Leo I of Halych in 1247 # Margaret of Hungary, canonized by the Roman Catholic Church in 1943, for whom Margaret Island in Budapest is named, having been the place where a royal monastery was established by her parents for her.

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.

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.

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.

Because of the following more and more chaotic internal situation after his death many thought him as the last ruler who brought peace to the realm. The epigram on his tomb refers this idea: Aspice rem caram: tres cingunt Virginis aram: Rex, Dux, Regina, quibus adsint Gaudia Trina Dum licuit, tua dum viguit rex Bela, potestas, Fraus latuit, pax firma fuit, regnavit honestas.

Béla was determined to regain the western part of Hungary which had been seized by Frederick II of Austria 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 war with Otakar II of Bohemia to gain control of Austria and Styria, but he finally had to give up all claims after a defeat in the first battle of Marchfeld (or battle of Kroisenbrunn) in 1260. He was regularly engaged in protecting the outer extremities 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 crowned to junior king of Hungary, setting up his own capital, and adopting foreign policies directly contrary to those of his father.

*Parsons, John Carmi. Medieval Queenship, 1997