The Battle of Magersfontein (/ˈmɑːxərsfɒnteɪn/ MAH-khərss-fon-tayn) was fought on 11 December 1899, at Magersfontein, near Kimberley, South Africa, on the borders of the Cape Colony and the independent republic of the Orange Free State (now in Sol Plaatje Local Municipality, Northern Cape). British forces under Lieutenant General Lord Methuen were advancing north along the railway line from the Cape to relieve the siege of Kimberley, but their path was blocked at Magersfontein by a Boer force that was entrenched in the surrounding hills. The British had already fought a series of battles with the Boers, most recently at Modder River, where the advance was temporarily halted.

Lord Methuen failed to perform adequate reconnaissance in preparation for the impending battle and was unaware that Boer Vecht-generaal (Combat General) De la Rey had entrenched his forces at the foot of the hills, rather than the forward slopes, as was the accepted practice. That allowed the Boers to survive the initial British artillery bombardment, and when the British troops failed to deploy from a compact formation during their advance, the defenders inflicted heavy casualties. The Highland Brigade suffered the worst casualties, and on the Boer side, the Scandinavian Corps was destroyed. The Boers attained a tactical victory and succeeded in holding the British in their advance on Kimberley. The battle was the second of three battles during what became known as the Black Week of the Second Boer War: Stormberg on Sunday 10 December, Magersfontein on Monday 11 December and Colenso on Friday 15 December 1899.

Following their defeat, the British delayed at the Modder River for another two months while reinforcements were brought forward. General Lord Roberts was appointed Commander in Chief of the British forces in South Africa and moved to take personal command of the front. He subsequently lifted the siege of Kimberley and forced Cronje to surrender at the Battle of Paardeberg.

In the early days of the war in the Cape Colony, the Boers surrounded and laid siege to the British garrisons in the towns of Kimberley and Mafeking and destroyed the railway bridge across the Orange River at Hopetown. Substantial British reinforcements (an army corps under General Redvers Buller) arrived in South Africa and were dispersed to three main fronts. While Buller himself advanced from the port of Durban in Natal to relieve the besieged town of Ladysmith and a smaller detachment under Lieutenant General Gatacre secured the Cape Midlands, the reinforced 1st Division under Lord Methuen advanced from the Orange River to relieve Kimberley.

Methuen advanced along the Cape–Transvaal railway line because a lack of water and pack animals made the reliable railway an obvious choice. Also, Buller had given him orders to evacuate the civilians in Kimberley and the railway was the only means of mass transport available. But his strategy had the disadvantage of making the direction of his approach obvious. Nevertheless, his army drove the Boers out of their defensive positions along the railway line at Belmont, Graspan, and the Modder River, at the cost of a thousand casualties. The British were forced to stop their advance within 16 miles (26 km) of Kimberley at the Modder River crossing. The Boers had demolished the railway bridge when they retreated, and it had to be repaired before the army could advance any further. Methuen also needed several days for supplies and reinforcements to be brought forward, and for his extended supply line to be secured from sabotage. The Boers were badly shaken by their three successive defeats and also required time to recover. The delay gave them time to bring up reinforcements, to reorganise, and to improve their next line of defence at Magersfontein.

After the Battle of the Modder River, the Boers initially retreated to Jacobsdal, where a commando from Mafeking linked up with them. The following day, Cronje moved his forces 10 miles (16 km) north to Scholtz Nek and Spytfontein, where they began to fortify themselves in the hills that made up the last defensible position along the railway line to Kimberley. Although closer to the British camp than the Boer camp, Jacobsdal was left poorly defended, and continued to function as the Boers' supply base until 3 December.

The Free State government decided to reinforce Cronje's position after the Battle of Belmont. Between eight hundred and a thousand men of the Heilbron, Kroonstad and Bethlehem commandos arrived at Spytfontein from Natal, accompanied by elements of the Ficksburg and Ladybrand commandos from the Basuto border. Reinforcements were also brought up from the Bloemhof and Wolmaranstad commandos who were besieging Kimberley. The remainder of Cronje's force arrived from the siege of Mafeking. Their force now numbered 8,500 fighters, excluding camp followers and the African labourers who performed the actual work of digging the Boer entrenchments.

Koos de la Rey had been absent from the army immediately after the Battle of the Modder River, having gone to Jacobsdal to bury his son Adriaan, who had been killed by a British shell during the battle. He arrived at the defensive positions on 1 December and surveyed the Boer lines the following day. He found the defences lacking, and realised that Cronje's position at Spytfontein was vulnerable to long range artillery fire from the hills at Magersfontein. He therefore recommended that they should move their defensive position forward to Magersfontein, to deny the British this opportunity. Cronje, who was the more senior officer, disagreed with him, so De la Rey telegraphed his objections to President Martinus Theunis Steyn of the Orange Free State. After consulting with President Paul Kruger of the Transvaal, Steyn visited the front on 4 December at Kruger's suggestion. Steyn also wished to settle a rift that had developed between the Transvaal and Free State Boers over the poor performance of his Free Staters in the battle on 28 November. He spent the next day touring the camps and defences, then summoned a krijgsraad (council of war).