Against Charlotte Aglaé's wishes, the Regent accepted an offer of marriage for his daughter that was proffered by Rinaldo d'Este, Sovereign Duke of Modena for his son and heir, Prince Francesco d'Este. Earlier projects to marry Charlotte Aglaé to either an English prince or to Charles Emmanuel III of Sardinia had failed. Her grandmother is known to have written to Charlotte Aglaé's aunt, Anne Marie d'Orléans, Queen of Sicily, on the marriage proposal. As a pre-condition to the liberation of Richelieu, her lover, it was decided that she would marry the heir of Modena.
According to her grandmother's writings, her future husband had fallen in love with the young Charlotte Aglaé upon, "the mere sight of her portrait".
Few expected the marriage to succeed, as Charlotte Aglaé had no desire to leave France. Her distant cousin, Marguerite Louise d'Orléans, who had previously been wedded against her will to Cosimo III de' Medici, Grand Duke of Tuscany in 1661, had suffered through a disastrous marriage. Eventually, Marguerite was forced to return to France in disgrace. People assumed that the same fate awaited Charlotte Aglaé. The Grand Duchess noticed the similarities between herself and her younger cousin and, unable to deal with the situation, refused to speak to Charlotte Aglaé about her impending nuptials:
"the Grand Duchess of Tuscany says that she will not see Mademoiselle de Valois nor speak to her, knowing very well what Italy is, and believing that Mademoiselle de Valois will not be able to reconcile herself to it. She is afraid that if her niece should ever return to France they will say, "There is the second edition of the Grand Duchess".
The original date for the marriage was 25 January 1720, but this date was postponed until the next month due to an oversight by the Bishop of Modena. Despite this, the marriage certificate was signed on 31 January.
On 11 February 1720, a proxy marriage was performed at the Tuileries Palace. Her brother, the Duke of Chartres stood in for his future brother-in-law, while her younger sister Louise Élisabeth held her train. After this, there was a banquet at the Palais Royal where the young king Louis XV attended and presented his gifts to the new Hereditary Princess of Modena.
Arriving in Reggio on 20 June, she met her father-in-law, husband and brother-in-law for the first time. The formal wedding ceremony took place on 21 June 1720 in Modena. Charlotte Aglaé received an enormous dowry of 1.8 million livres, half of which was contributed in the name of the young king, Louis XV, on orders of the Regent. From her adopted country, Charlote Aglaé received a trousseau consisting of diamonds and portraits of her future husband.
In Modena, Charlotte Aglaé grew bored and lonely. Her father-in-law, with whom she got on rather well, was a dull man ruled by monks and his favourites. The court at Modena was said to have been more of a convent then a court.
In order to distract herself from the boring court, Charlotte Aglaé began holding small private gatherings in her apartments where she would entertain a small circle attended by her husband's three surviving sisters.
In September 1720, Charlotte Aglaé caught smallpox and, according to her grandmother, the last sacrements were administered. She called her French confessor, Colibeaux, to her bedside, and, handing him a casket, directed him to secretly burn all the papers which it contained. Probably among these papers were the love-letters which she had received from the Duke of Richelieu. During this illness, her husband was forbidden to see her; he stayed at his villa at Sassuolo until she recovered. Soon the marriage was criticised for not having produced any children, and blame fell upon the then recovering princess.
This criticism of the couple led them to flee to Verona for a short spell, much to the annoyance of her father-in-law. In retaliation, he cut off the postal service to the prince and princess in the hope that they would return. It was at this time that Charlotte first asked her father to let her return to France and live at Versailles with her family. This request only served to complicate marriage plans already under way for her younger sisters Louise Élisabeth and Philippine Élisabeth. In December 1723, while she was away in Italy, her father died. Her younger brother, Louis, succeeded to the Orléans titles.
Following her father's death, Charlotte Aglaé and her husband were asked to stay at the prince's villa at Reggio, in a sort of private exile away from the court of Modena.
In 1727, her former lover, the Duke of Richelieu, visited her in Modena in disguise, and the two resumed their romantic affair. When this was exposed, her husband allowed her to return to France in temporary disgrace. She returned to Modena later in 1727. The next year, 1728, she and her husband fled to Genoa; five years later in 1733, she returned