Eustacia María Caridad del Río Hernández (Russian: Каридад Рамоновна Меркадер, romanized: Karidad Ramonovna Merkader; 29 March 1892 – 1975), better known as Caridad del Río, Caridad Mercader or Caritat Mercader, was a Spanish communist militant and an agent of the Soviet NKVD. She is also known for being the mother of Ramón Mercader, the assassin of Leon Trotsky, and for having personally participated in the operation.

Caridad Mercader belonged to a wealthy family from Barcelona of Indiano origin (term applied to a Spaniard who emigrated to the Americas who later returned to Spain enriched) in the early 20th century. She married Pablo Mercader, a member of Barcelona's industrial upper class, from whom she took the name (Spanish women do not normally take their husbands' surnames), and with whom she had five children. After the end of her marriage to Pablo Mercader, she moved away from her family and permanently turned her back on the social class they represented. This decision was motivated in part by an episode of forced institutionalization during which she was subjected to electroshock therapy and her former husband's attempts to change her state of "sexual apathy" through visits to local brothels. Mercader began to frequent anarchist circles and soon embraced communist ideology. At the outbreak of the Spanish Civil War, she participated in the fights against the military uprising in Barcelona and joined the groups that left for Aragon, where she suffered severe injuries during an aerial attack.

Mercader achieved some notoriety as a member of the Unified Socialist Party of Catalonia (Partit Socialista Unificat de Catalunya, PSUC). In 1936 she led a propaganda mission to Mexico and later became an agent of the NKVD in Spain. Her son Ramón, also a member of the PSUC and an officer in the Spanish Republican Army, was also recruited by Soviet espionage during the war, likely with the involvement of his mother. Under orders from Josef Stalin, as part of Operation Utka (Operation Duck), Ramon Mercader was enlisted and trained to assassinate Leon Trotsky, who was in exile in Mexico. Caridad, who had settled in Paris some time in 1937, also participated in the operation. When Ramón was arrested after murdering Trotsky, Caridad managed to leave Mexico and escape to the Soviet Union, where she was received with honors, awarded the Order of Lenin. The Hero of the Soviet Union was reserved for Ramon upon his release from a Mexican prison. In the Soviet Union, Caridad actively participated in conflicts between the different factions of exiled Spanish communists, including with Dolores Ibárruri, La Pasionaria.

Caridad found conditions in the Soviet Union disappointing and never adapted to life there. She bitterly told her son Luis and confidante Enrique Castro Delgado that they had fought for "Utopia" but were living in "Hell". She expressed that she felt deluded and that she had turned her son Ramon into a murderer, her son Luis into a hostage, and her other two children into ruins. She felt their only recompense had been "cuatro porquerias" (four pieces of trash), referring to the medals. In 1944, with some difficulty, Caridad obtained a permit to leave the Soviet Union. Violating the agreed conditions that she settle in Cuba, she traveled to Mexico, with the aim of achieving the release of her son Ramón. Unknown to Caridad, at Stalin's direction, the Soviets were running an undercover operation to stage the prison escape of Ramón Mercader. The awkward intervention of Caridad Mercader was counterproductive, causing the Mexican authorities to toughen Ramón's prison conditions and the Soviets to abandon their operation. Ramón was left in prison to serve out the remaining 16 years of his 20-year sentence. Ramón, who according to his brother Luis never shared his mother's passion for the communist cause, blamed his mother for botching his release and never forgave her interference.

After the failure of Operation Utka, Caridad settled in Paris, where her daughter Montserrat and son Jorge lived with their families, enjoying a Soviet pension. Disillusioned with communist reality, she nevertheless stubbornly continued to be a communist, worshiping Stalin and believing in his doctrine. She occasionally traveled to the Soviet Union to visit her sons, Luis, as well as Ramón, who had settled there after serving his sentence in Mexico. Caridad Mercader died in the French capital in 1975. The Soviet embassy in Paris took care of the funeral and burial.

Eustacia María Caridad del Río Hernández was born in Santiago de Cuba in 1892 into a wealthy family. Her father, Ramón del Río, was originally from the Spanish province of Santander, but when the family decided to return to Spain, a few years before Cuban independence, they settled in Barcelona, where they became part of the city's social elite. Caridad had at least two brothers. Although Caridad later claimed that her father had been governor of Santiago as well as the first planter to free his slaves on the Antillean island, none of this is true. Nor, contrary to what Caridad asserted, had her mother sympathized with the Cuban independence movement.

Caridad studied at the Catholic school of the Sacred Heart of Sarriá, also spending time in the centers that the congregation had in Paris and London. As a result, she spoke French and English perfectly. Apparently, during her adolescence she felt the call of a religious vocation, although she did not choose it.

When Caridad was barely 16 years old, on June 13, 1908, the Barcelona press announced the engagement of Caridad del Río to Pablo Mercader Marina, seven years older than her and a member of a prosperous family in the textile business. Pablo's father, Narciso Mercader Sacanella, had started his business with a factory in Badalona, expanding it with several more factories in Barcelona. The engagement was a bond that united two wealthy families of the Barcelona bourgeoisie. Both of the fiancés were equestrians. Caridad herself later stated that she fell in love with Pablo Mercader for his mastery as a rider. On January 7, 1911, the wedding took place. Mercader had an affable character, politically aligning himself with conservative Catalan nationalism and having been a member of the somatén. At that time, Caridad "was a beautiful teenager with a round face and pleasant features with a sweet look [...] in her green eyes that were always her most distinctive feature." The couple settled on Illas i Vidal street, in the upper-class neighborhood of Sant Gervasi de Cassoles. The young wife adopted her husband's surname and would thereafter be known as Caridad, or Caritat, Mercader. The couple had five children: Jorge (b. 1911), Ramón (b. 1913), Montserrat (b. 1914 ), Pablo (b. 1915) and Luis (b. 1923). Shortly after Pablo's birth, Jorge fell ill with poliomyelitis and suffered paralysis of both legs.