Ras Mengesha Yohannes (Tigrinya: መንገሻ ዮሓንስ; 1868 – 1906) was governor of Tigray and a son of Emperor Yohannes IV (r. 1872–89). His mother was Welette Tekle Haymanot wife of dejazmach Gugsa Mercha. Ras Araya Selassie Yohannes was his younger half brother. Prior to the Battle of Metemma, Mengesha Yohannes was considered to be a nephew of Emperor Yohannes IV. During the battle, the Emperor was mortally wounded and it was on his deathbed that Mengesha Yohannes was acknowledged as his "natural" son and designated as his heir. This created something of a succession problem.

Fighting between various relatives of the slain Emperor split his camp and prevented Mengesha from making a viable bid for the Imperial throne. Instead, the throne was assumed by Negus Menelik of Shewa. Ras Mengesha refused to submit to Menelik and later even flirted with joining the new Italian colony of Eritrea. He hoped that the Italians would support his rebellion against Emperor Menelik. However, encroachments by the Italians into his native Tigray, their previous enmity to his father Yohannes, and recognition that the ultimate goal of the Italians was to conquer Ethiopia themselves, led Mengesha Yohannes to finally submit to Menelik II. On 2 June 1894, he and his three major lieutenants went to the new capital at Addis Ababa. Within the newly constructed reception hall of the Grand Palace, the Emperor awaited them. He was seated on his throne with a large crown on his head. Mengesha Yohannes and his lieutenants each carried a rock of submission on his shoulder. They approached, prostrated themselves, and asked for forgiveness. Menelik simply declared them pardoned.

Following their allegiance with Menelik, they returned to Tigray, where Bahta Hagos initiated the rebellion against the Italians. Mengesha then led his army against the Italians at the Battle of Coatit, where his force was rebuffed. Another skirmish led by Fitawrari Gebeyehu, the vanguard commander of Menelik forces on the way to Adwa, annihilated the Italians at Amba Alagi. The war culminated in 1896, as Mengesha Yohannes and the forces of Tigray fought at the side of Menelik against the Italians at the pivotal Battle of Adwa. In 1899, Mengesha Yohannes rebelled again against Menelik when he was denied the title of Negus of Zion (his descendants would be outraged decades later when Ras Mikael of Wollo was crowned with this title by Lij Iyasu). Emperor Menelik had Ras Mengesha captured and put under house arrest at the old Shewan royal palace at Ankober.

Before 1889, Mengesha Yohannes was considered the Emperor's nephew, but on 9 March 1889 the Emperor on his deathbed, in Metemma, recognized him as his son, from a side-relation with his brother Gugsa's wife. Therefore, some informants still claim that Mengesha Yohannes was strongly attached to Yohannes throughout his life. Dejazmach Mengesha Yohannes was designated by the Emperor as his heir-to-the-throne, and allegedly crowned still on the battlefield of Metemma, but Mengesha Yohannes was not able to claim the throne. In the aftermath of the disastrous battle of Metemma, the Tigrayan ruling families found themselves embroiled in a power struggle that led to a regional civil war. This state of anarchy sapped the energy of Mengesha Yohannes's camp to unite the region under his authority. Quickly, instead atse Menelik II was proclaimed emperor in November 1889.

Ras Mengesha Yohannes, however, had the strong military and political backing of Yohannes's chief military general ras Alula Engada. Additionally, he had inherited a considerable number of armaments and soldiers that had long bolstered the power of Yohannes IV. However, historians cast doubts on Mengesha Yohanens's basic military prowess and leadership skills to mobilize the available resources and prevail over rivals, in contrast to his predecessor Yohannes. Mengesha Yohannes's strong political ambition once nearly pushed him to ally with the Italians, as demonstrated by the Mereb Convention of 1891. By 1890, the Italians had taken control of the Mereb Mellash, and aimed at the control of entire Tigray. In the meanwhile, Mengesha Yohannes and his Tigrayan vassals had revolted against atse Menelik II. Italian attempts to instigate them were, nonetheless, short-lived. The Italians' non-committed stance, coupled with Mengesha Yohannes's hesitant attitude finally convinced him to end his rebellion, even against the opposition of his counsellors and his military supporters, such as ras Alula.

In June 1894, Mengesha Yohannes ultimately dropped his claims to the imperial throne and officially submitted to atse Menelik II, ostensibly in expectation of the title of negus of Tigray. Menelik, in turn, appointed him governor, not negus of Tigray. For a while, Mengesha Yohannes demonstrated his loyalty. The conflict subsequently developing between the Italians and Menelik, gave him the opportunity to prove his indispensability to the central government.

During the first phase, in the years 1894–95, the Italians made a number of incursions into northern Ethiopia. Mengersha Yohannes was then responsible for defending the northern borders. He was beaten in the two battles of Kweatit (1894), Senafe (Eritrea) and Debre Hayle (1895): in early 1896 he even retreated from his stronghold Mekelle, but kept the Italians busy until a united Ethiopian army was gathered to face them.

Ras Mengesha Yohannes played an even more important role in the next phase of the conflict. Together with ras Alula, he was largely responsible for organizing and running the intelligence of Menelik II. He exploited his local connections, gathered intelligence and rendered a fairly outstanding command of the Tigrayan army. All these inputs had a crucial role in deciding the outcome of the battle of Adwa in 1896, which ended up as a grim loss to the Italians.