Matilda of Hainaut (French: Mathilde de Hainaut; November 1293 – 1331), also known as Maud and Mahaut, was Princess of Achaea from 1316 to 1321. She was the only child of Isabella of Villehardouin and Florent of Hainaut, co-rulers of Achaea from 1289 to 1297. After Florent's death in 1297, Isabella continued to rule alone until she remarried to Philip of Savoy in 1300. Per arrangements made with King Charles II of Naples, Isabella was not allowed to marry without his consent and after Philip failed to adequately participate in the king's campaigns against Epirus, Charles in 1307 revoked their rights to Achaea. Matilda, just fourteen years old, tried to press her claim as their heir but was refused by the bailiff Nicholas III of Saint Omer, who instead chose to wait for orders from Naples. Shortly thereafter, Charles appointed his favorite son, Philip of Taranto as the new Prince of Achaea.

Philip of Taranto spent little time in Greece and appointed as his bailiff Guy II de la Roche, Matilda's husband (who she had married at age 7, about 7 years earlier). Guy did not last long in this position, dying in 1308. Matilda was then betrothed by Philip of Taranto to his eldest son, Charles of Taranto. In 1313, this betrothal was broken off and Matilda was married to Louis of Burgundy as compensation to the House of Burgundy due to Philip of Taranto in the same year having married Catherine of Valois, previously betrothed to Hugh V, Duke of Burgundy. Philip of Taranto also renounced his rulership of Achaea and bestowed the Principality of Achaea on Matilda and Louis. Before they had travelled to their new domain, Achaea was seized in 1315 by the usurper Ferdinand of Majorca. Matilda and Louis landed in Achaea in early 1316 and secured control of the principality after the defeat and death of Ferdinand in the Battle of Manolada. Their co-rule did not last long; Louis died less than a month later, widowing Matilda (now 22-years-old) for the second time.

The new king of Naples, Robert, wished to exploit Matilda's uncertain position to gain the principality back for his family. In 1317, he proposed that Matilda should marry his brother, John of Gravina. Matilda refused as she did not wish to enter into a third political marriage. In 1318, Robert's emissaries abducted the princess and brought her to Naples by force. She was forced to go through a marriage ceremony with John, but she refused to recognize him as her husband. In 1321, Matilda was dragged in front of Pope John XXII who ordered her to obey Robert and marry John, but she still refused. Matilda then confessed that she had secretly married the knight Hugh de la Palisse. No more attempts were made to marry her to John but Robert could now revoke Achaea from her control as she had married without his consent. Robert also fabricated a story that Hugh had made an attempt on his life, and that Matilda was his accomplice, and used this as an excuse to imprison the princess. Matilda spent the rest of her life as a prisoner, first in the Castel dell'Ovo in Naples and then in Aversa, where she died in 1331.

Born in November 1293, Matilda of Hainaut was the only child of Isabella of Villehardouin and Florent of Hainaut, who ruled the Principality of Achaea together from 1289 to 1297. Matilda's mother was the eldest daughter of William of Villehardouin (Prince of Achaea 1246–1278). William's death in 1278 rendered the male line of the Villehardouin family extinct. As part of a marriage between Isabella and Philip of Sicily, a younger son of King Charles I of Naples, William acknowledged Charles and Charles's descendants as his heirs even if Isabella and Philip had no children. Philip predeceased Charles and died childless, which meant that Charles succeeded William as prince in 1278 without opposition. By 1289, Charles I's son and successor, King Charles II, presided over an economically poor and politically tumultuous Achaea. Upon the advice of some local barons, he arranged the marriage of Isabella and Florent and granted the principality to them as his vassals. Charles II's only condition was that if Florent died, Isabella or any daughters were not to remarry without royal consent: if they did, the principality was to revert to the king.

Upon the death of Florent in early 1297, Matilda's mother became the sole ruler of the principality. Matilda was three years old at the time of her father's death. In order to safeguard the principality through establishing a marriage alliance, the young princess was soon married off to Guy II de la Roche, the Duke of Athens, who had only recently come of age. The three-year-old Matilda was sent to Athens, but Charles II did not accept the marriage and on 3 July 1299 he reminded Guy that Matilda was not allowed to marry without royal consent, ordering that she be sent back to Achaea. Isabella and Guy had already appealed to Pope Boniface VIII to sanction the marriage since Isabella and Guy's mother Helena Angelina Komnene were cousins. After the Pope sanctioned the marriage on 9 August 1299, there was little Charles II could do to stop it and the king thus also gave his consent on 20 April 1300.

Isabella did not remain a widow for long: also in 1300, she met with Philip of Savoy, supposedly a valiant knight, in Rome (although negotiations had apparently been going on for some time) and shortly thereafter married him. The marriage was supported by Pope Boniface VIII. Though Charles II initially objected and tried to appeal to the 1289 agreement with Isabella and Florent, he eventually relented and reluctantly invested Philip as Prince of Achaea. Relations between Charles and Philip, which had not been off to a good start, worsened over the years due to Philip's reluctance, and at times outright refusal, to aid the kings in his wars against the Despotate of Epirus. As a result, Charles resolved to appeal not only to the marriage between Isabella and Philip having happened without his consent but also to Philip's breach of the feudal code and thus in 1307 revoked the rights of both Philip and Isabella to the principality. Philip had foreseen this and had shortly before returned to the Savoyard state, leaving the bailiff Nicholas III of Saint Omer in charge.

Upon the deposition of her mother and step-father, Matilda, approximately fourteen years old, tried to claim the principality for herself but Nicholas refused her and instead chose to wait for orders from Charles. Matilda's husband Guy II was outraged and as revenge captured portions of the city of Thebes that had been held by Nicholas. Charles ignored the claim of Matilda and instead bestowed the principality on the favorite of his younger sons, Philip of Taranto. In order to ensure that Isabella and Philip of Savoy did not try to regain Achaea, Charles and his son bribed them through offering them the fief of Alba in Italy, on the shores of the Fucine Lake, as compensation. Isabella was reluctantly forced to accept these terms and died not long after, in 1312. Her will, written in 1311, outright disregarded the revocation of Achaea and designated Matilda as the "heiress of all Achaea", with the exception of a handful of castles that Isabella left to her own sister, Margaret of Villehardouin. Philip of Taranto did not stay in Achaea for long, leaving to wage an unsuccessful campaign against Epirus and then returning to Naples. As his bailiff (as with Nicholas before him effectively a regent) he named Guy, Matilda's husband. Guy's position as the de facto ruler of Achaea was strengthened by his marriage to Matilda, granting a certain dynastic legitimacy, and by him at this point being the most powerful feudal lord in Greece.

Guy did not enjoy this position for long, as he died of illness already on 8 October 1308, the last of his line, making the young Matilda a widow. In order to ensure that Matilda would not attempt to enforce her claim to Achaea against himself, Philip of Taranto betrothed her to his eldest son, Charles of Taranto.