Yostos or Justus ( Ge'ez ዮስቶስ, throne name Tsehay Sagad Ge'ez ፀሓይ ሰገድ, "to whom the sun bows") was nəgusä nägäst (14 October 1711 – 19 February 1716) of Ethiopia.
According to James Bruce, he was the son of Delba Iyasu and a daughter of Emperor Iyasu I. He had served as governor of Samien under Emperor Tekle Haymanot, but fell out of favor under Emperor Tewoflos—despite it was Yostos who, immediately after the assassination of Emperor Tekle Haymanot travelled to Mount Wehni and brought Tewoflos down.
According to Richard Pankhurst, on the death of Tewoflos, the chief nobles of Ethiopia feared that the cycle of vengeance that had characterized the reigns of the previous two rulers would continue if a member of the Solomonic dynasty were picked for the throne, so they selected one of their own to be nəgusä nägäst. However Yostos encountered many challenges to his authority, and was forced to remain in the capital of Gondar for his entire reign, leaving the city only to hunt. After a few years, the political situation stabilized enough for him to construct two new churches in Gondar, Lideta Mariyam ("Birth of the Virgin Mary") in 1713, and Abba Antons ("Father Anthony") in 1715.
Bruce notes that Yostos faced a conspiracy to depose him shortly after taking the throne: while he was away from Gondar on a hunt, a group of men he had entrusted his government to had planned to overthrow him. Yostos, with a body of picked men, returned to his capital at night and surprised them sitting in council. His chief minister, Ras Hezekias, and his Master of his Household, Heraclides, along with five others, had their nose and ears cut off then thrown into prison. One of the chief conspirators, Benaia Basile, however managed to escape, having been warned of Yostos' sudden return, but was later caught and punished.
The following year Yostos led a slave-raiding expedition against one of the peoples, once collectively referred to as the Shangalla, who lived along the western border of his kingdom, specifically the people known as the Baasa. These he surprised, slaying the adults and taking their children captive. Yostos had his expedition brought to an end after this first predation when the death of his confidant Ras Fasa Krestos.
In January 1716 Yostos grew ill, and according to his Royal Chronicles withdrew from public life. Bruce provides more information: while supervising the work on the Abba Antons church Yostos "was taken suddenly ill, and, suspecting some unwholesomeness or witchcraft in his palace, ordered his tent to be pitched without the town until his apartments should be smoked with gunpowder." However, the fumigation accidentally burned down part of the palace, which was seen as a "very bad omen". Yostos, still unwell, took up residence in another part of the Royal Enclosure, but fear that he would make his son Fasil heir to the throne led to a battle between his courtiers (who wanted the ailing Emperor to proclaim an heir) and the Imperial Guard (who were loyal to the Solomonic dynasty). Victorious, the Imperial Guard proclaimed Dawit III Emperor 30 January. Meanwhile, Emperor Yostos was still alive in the palace, forgotten in his sick bed until his death. He was given a respectful burial in Lideta church.