Zara Yaqob (Ge'ez: ዘርዐ ያዕቆብ;[a] 1399 – 26 August 1468) was Emperor of Ethiopia, and a member of the Solomonic dynasty who ruled under the regnal name Qostantinos I (ቆስጠንጢኖስ; "Constantine").

He is known for the Geʽez literature that flourished during his reign, the handling of both internal Christian affairs and external wars with Muslims, along with the founding of Debre Birhan as his capital.

[3] 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.

"[4] 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.

[5][6] 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.

[7] 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.

Thrust into a position of leadership "with no experience of the affairs of state, he [Zara Yaqob] was faced by a kingdom seething with plots and rebellions, a Church riven with heresies, and outside enemies constantly threatening invasion."

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.

[1] 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.

[14] Zara Yaqob invaded Hadiya after they failed to pay the annual tribute exacted upon them by the Ethiopian Empire, and married its princess Eleni, who was baptized before their marriage.

It is during this time that the title of the coastal regions' ruler, Bahr Negash, first appears in records and according to Richard Pankhurst the office was likely introduced by Zara Yaqob.

[22] After observing a bright light in the sky (which most historians have identified as Halley's Comet, visible in Ethiopia in 1456), and believing it to be a sign from God, indicating His approval of the execution by stoning of a group of heretics 38 days earlier, Zara Yaqob established Debre Berhan as his capital for the duration of his reign.

[28] Alfonso signed a letter addressed to “Zere Jacobo, son of David of the house of Solomon, Ethiopian Emperor, our very dear friend and brother” and dated 18 September 1450.

He lamented the earlier loss of "thirteen men, masters in different arts," who had died on their journey after being dispatched at the request of Zara Yaqob’s predecessor Yeshaq in 1427.