Zara Yaqob (Ge'ez: ዘርዐ ያዕቆብ; 1399 – 26 August 1468) was Emperor of Ethiopia, and a member of the Solomonic dynasty who ruled under the regnal name Constantine I (Ge’ez: ቈስታንቲኖስ, ḳʷästantinos; Amharic: ቆስጠንጢኖስ, ḳosṭänṭinos). He is known for the Geʽez literature that flourished during his reign, the handling of both internal Christian affairs and external wars with Muslims, and the founding of Debre Birhan as his capital. He reigned for 34 years and 2 months (r. 1434-1468).

The British historian Edward Ullendorff stated that Zara Yaqob "was unquestionably the greatest ruler Ethiopia had seen since Ezana, during the heyday of Aksumite power, and none of his successors on the throne – excepted only the emperors Menelik II and Haile Selassie – can be compared to him."

Born at Telq in the province of Fatagar, Zara Yaqob hailed from the Amhara people. He was the youngest son of Emperor Dawit I by his wife, Igzi Kebra. His mother Igzi lost her first son, and, having been sick during her second pregnancy, prayed fervently to the Virgin Mary to keep her new child alive. She later gave birth to Zara Yaqob, who later recorded this as a miracle in the Ta'ammara Maryam, one of Zara Yaqob's chronicles written in Amharic.

Paul B. Henze repeats the tradition that the jealousy of his older brother Emperor Tewodros I forced the courtiers to take Zara Yaqob to Tigray where he was brought up in secret, and educated in Axum and at the monastery of Debre Abbay. While admitting that this tradition "is invaluable as providing a religious background for Zara Yaqob's career", Taddesse Tamrat dismisses this story as "very improbable in its details". The professor notes that Zara Yaqob wrote in his Mashafa Berhan that "he was brought down from the royal prison of Mount Gishan only on the eve of his accession to the throne."

Upon the death of Emperor Dawit, his older brother Tewodros ordered Zara Yaqob confined on Amba Geshen (around 1414). Despite this, Zara Yaqob's supporters kept him a perennial candidate for Emperor, helped by the rapid succession of his older brothers to the throne over the next 20 years, which left him as the oldest qualified candidate. David Buxton points out the effect that his forced seclusion had on his personality, "deprived of all contact with ordinary people or ordinary life." Thrust into a position of leadership "with no experience of the affairs of state, [Zara Yaqob] was faced by a kingdom seething with plots and rebellions, a Church riven with heresies, and outside enemies constantly threatening invasion." Buxton continues,

Although he became Emperor in 1434, Zara Yaqob was not crowned until 1436 at Axum, where he resided for three years.

During his first years on the throne, Zara Yaqob launched a strong campaign against survivals of pagan worship and "un-Christian practices" within the church. According to a manuscript written in 1784, he appointed spies to search and "smell out" heretics who admitted to worshipping pagan gods such as Dasek, Dail, Guidale, Tafanat, Dino and Makuawze. These heretics were decapitated in public. The spies also revealed that his sons Galawdewos, Amda Maryam, Zar'a Abraham and Batra Seyon, and his daughters Del Samera, Rom Ganayala and Adal Mangesha were heretics and thus they were all executed as a result. He then issued a royal edit ordering every Christian to bear on his forehead a fillet inscribed "Belonging to the Father, and the Son, and the Holy Spirit." And fillets had to be worn on the arms, that on the right being inscribed "I deny the Devil in [the name of] Christ God," and that on the left, "I deny the Devil, the accursed. I am the servant of Mary, the mother of the Creator of all the world." Any man who disobeyed the edict had his property looted and was either beaten or executed.

The Ethiopian Church had been divided over the issue of Biblical Sabbath observance for roughly a century. One group, which was loyal to the Coptic Orthodox Church of Alexandria, believed that the day of rest should be observed only on Sunday, or Great Sabbath. Another group, the followers of Ewostatewos, believed with its founder that both the original seventh-day Sabbath (Saturday, or Lesser Sabbath) and Sunday should be observed. Zara Yaqob was successful in persuading two recently-arrived Egyptian Abuna, Mikael and Gabriel, into accepting a compromise aimed at restoring harmony with the House of Ewostatewos, as the followers of Ewostatewos were known. At the same time, he made efforts to pacify the House of Ewostatewos. While the Ewostathians were won over to the compromise by 1442, the two Abuns agreed to the compromise only at the Council of Debre Mitmaq in Tegulet (1450).