Joanna I of Naples, also known as Johanna I (Italian: Giovanna I; December 1325 – 27 July 1382), was Queen of Naples, and Countess of Provence and Forcalquier from 1343 to 1381; she was also Princess of Achaea from 1373 to 1381.

Joanna was the eldest daughter of Charles, Duke of Calabria and Marie of Valois, Duchess of Calabria to survive infancy. Her father was the son of Robert, King of Naples, but he died before his father in 1328. Three years later, King Robert appointed Joanna as his heir and ordered his vassals to swear fealty to her. To strengthen Joanna's position, he concluded an agreement with his nephew, King Charles I of Hungary, about the marriage of Charles's younger son, Andrew, and Joanna. Charles I also wanted to secure his uncle's inheritance for Andrew, but King Robert named Joanna as his sole heir on his deathbed in 1343. He also appointed a regency council to govern his realms until Joanna's 21st birthday, but the regents could not actually take control of state administration after the King's death.

Joanna's personal life crucially affected the political stability of the Kingdom of Naples (murder of her first husband Andrew in 1345, the invasions of King Louis I of Hungary — justified as avenging the death of his brother — and her three later marriages with Louis of Taranto, James IV, titular King of Majorca and Otto, Duke of Brunswick-Grubenhagen) and undermined her position with the Holy See; moreover afterwards, during the Western Schism, she chose to support the Avignon Papacy against Pope Urban VI, who in retaliation declared her a heretic and usurper on 11 May 1380.

With all her children having predeceased her, Joanna's heirs were the descendants of her only surviving sister Maria, whose first marriage with their cousin Charles, Duke of Durazzo was performed without her permission, becoming both spouses in the heads of the political faction against Joanna. Trying to reconcile with the Durazzo branch and with the purpose of securing her succession, Joanna arranged the marriage of her niece Margaret of Durazzo with her first cousin (and Joanna's second cousin) Charles of Durazzo, who eventually captured and imprisoned Joanna, and finally ordered her assassination on 27 July 1382.

Joanna was the eldest surviving child of Charles, Duke of Calabria (only surviving son of Robert, King of Naples, and Marie of Valois (sister of King Philip VI of France). The Renaissance historian Donato Acciaioli claimed that she had been born in Florence, but she may have actually been born during her parents' travel towards the town, according to scholar Nancy Goldstone. The precise date of Joanna's birth is unknown; according to some researchers, she was most probably born in 1326 or 1327; however, given the fact that her mother gave birth to four or five children during her four years' marriage, Joanna was probably born in either December 1325 or early 1328. The early deaths of two of her siblings — A daughter, variously named Eloisa, Louise or Maria (born in January/February 1325; died 27 December 1325) and Charles Martel (born 13 April 1327; died eight days later) — left Joanna as the second in line to the Neapolitan throne after her own father.

Charles of Calabria died unexpectedly on 9 November 1328. With his death, his father faced the problem of succession, because Charles' posthumous child was also a daughter, Maria, born on 6 May 1329. Although Neapolitan law did not prevent women from inheriting the throne, the concept of a reigning queen was unusual. The agreement between the Holy See and Robert the Wise's grandfather, Charles I of Anjou, had explicitly acknowledged the right of Charles I's female descendants to inherit the throne, but it also stipulated that a female monarch was to marry and to allow her husband to rule. Furthermore, the Neapolitan royal house was a branch of the Capet dynasty of France and the French had recently excluded women from royal succession. Robert's nephew, Charles I of Hungary, had been disinherited in Robert's favor in 1296, but he did not abandon his claim to the Regno (or the Kingdom of Naples). Pope John XXII had ignored Charles's demands for years, but Robert's support for the Spiritual Franciscans (whom the Pope regarded as heretics) and his negligence to pay the yearly tribute to the Holy See gave rise to tensions between Naples and the Papacy. Robert's two younger brothers, Philip II, Latin Emperor, and John, Duke of Durazzo, could also claim the throne against a female monarch.

Robert was determined to secure the succession to his descendants and named Joanna and Maria as his heirs at a public ceremony at the Castel Nuovo in Naples on 4 December 1330. John of Durazzo and his wife, Agnes of Périgord, accepted Robert's decision (possibly in the hope that one of their three sons could marry Joanna), but Philip I of Taranto and his wife, Catherine of Valois, decided not to obey. When Joanna was invested with the right to succeed her grandfather on 30 November, John and Agnes were among the Neapolitan vassals who swore fealty to her, but Philip and Catherine did not attend the ceremony. Even the Pope could only persuade Philip to send a deputy to Naples to pay homage to Joanna on his behalf on 3 March 1331.

Charles I of Hungary had meanwhile asked the Pope to persuade Robert to restore the two fiefs that his father, Charles Martel, had held in the Regno—the Principality of Salerno and the Honor of Monte Sant' Angelo—to him and his sons. He also put forward a marriage alliance, asking Joanna's hand for one of his sons. The Pope supported the plan and kept urging Robert to accept it. The widowed Catherine of Valois approached her half-brother, Philip VI of France, to intervene and block the marriage. She proposed her sons, Robert, Prince of Taranto and Louis, as suitable husbands for Joanna and Maria. The Pope was resolute and issued a bull on 30 June 1331, ordering that Joanna and her sister were to marry Charles I's sons. Initially, Charles I's eldest son, Louis, was designated for husband to Joanna. His younger brother, Andrew, was only Louis's substitute in case of his premature death. At one point during the negotiations, Charles I changed his decision and appointed Andrew to marry Joanna.