The Wham Paymaster robbery (/ˈhwɑːm/ WHAHM) was an armed robbery of a United States Army paymaster and his escort on May 11, 1889, in the Arizona Territory. Major Joseph W. Wham was transporting a payroll consisting of more than US$28,000 (equivalent to $979,900 in 2024) in gold and silver coins from Fort Grant to Fort Thomas when he and his escort of eleven Buffalo Soldiers were ambushed. During the attack, the bandits wounded eight of the soldiers, forced them to retreat to cover, and stole the payroll.

As a result of their actions under fire, Sergeant Benjamin Brown and Corporal Isaiah Mays were awarded the Medal of Honor while eight other soldiers received a Certificate of Merit. Eleven men, most from the nearby Mormon community of Pima, were arrested, with eight of them ultimately tried on charges of robbery. All of the accused were acquitted, and the stolen money was never recovered.

In April 1889, Special Order 37 directed all paymasters in the District of Arizona to pay troops mustered as of April 30. Major Joseph Washington Wham, a U.S. Army paymaster, was assigned to Fort Bowie, Fort Grant, Fort Thomas, Fort Apache and Camp San Carlos. Wham and his clerk, William T. Gibbon, met a train carrying the payroll in Willcox on May 8. The paymaster performed his duties at Fort Bowie on May 9 and at Fort Grant on May 10.

Early on May 11, Major Wham left Fort Grant with two mule-drawn carriages, a covered ambulance and an open wagon, for the 46-mile (74 km) journey to Fort Thomas. The remaining payroll consisted of US$28,345 in gold and silver coins and weighed an estimated 250 pounds (110 kg).

The commander of Fort Grant had assigned eleven enlisted Buffalo Soldiers from the 24th Infantry and 10th Cavalry to serve as an escort between his fort and Fort Thomas. In addition to the military personnel, there was a civilian contractor who drove the open wagon. The two non-commissioned officers leading the escort were armed with revolvers while the privates carried single-shot rifles and carbines. Wham and the civilian members of the convoy were unarmed. Accompanying Wham on the journey was a black woman, Frankie Campbell (also known as Frankie Stratton), who was the wife of a soldier stationed at Fort Grant and was going to collect gambling debts owed to her and her husband by soldiers stationed at Fort Thomas.

There had never been an attack by highwaymen upon a paymaster within Arizona Territory prior to May 1889. Despite this, there were several factors favoring such an attack. At the time, the territory was remote and had only a small and scattered population. Many residents of Arizona Territory held the U.S. federal government in low regard, feeling it ruled the territory from Washington with little interest in the territorial residents' well-being. Compounding this was that most white residents of Arizona had been either Confederates or Confederate sympathizers during the American Civil War. Decades of hostilities between Washington and the Church of Jesus Christ of Latter-day Saints left hard feelings among the territory's Mormon population. Recent efforts to enforce the Edmunds Act, which made polygamy a felony, had increased anger among this group. Finally, strong racial biases held by the white population against black soldiers may have been exacerbated by Mormon teachings that placed blacks on a "lowly rung of the social ladder". This led to a situation where some area residents could have rationalized that the payroll would have been better spent supporting the local communities instead of in the hands of black soldiers, who were seen as likely to spend it on immoral pursuits.

The road to Fort Thomas went southwest to the town of Bonita before turning north and following the western side of Mount Graham. About 25 miles (40 km) from Fort Grant the road entered a pass leading to the Gila River valley. Wham's convoy reached Cedar Springs, in the pass, around noon and swapped their mules for a fresh set that were waiting at the NN ranch. Campbell, whose horse was faster than Wham's wagons, waited at a stagecoach half-way station a short distance further down the road. The station was operated by Mormon polygamist Wiley Holladay. Holladay was away that day, leaving his wives, Harriet and Eliza, to run the place. Wham and his escort reached the station around 12:45 pm, at which time Campbell rejoined the group.

The site of the attack was about 3 miles (5 km) north of the stagecoach station. The road at that point descended from a high ridge through a narrow gorge to a creek bed. The east side of the gorge consisted of a steep, rocky slope rising about 50 feet (15 m) above the road with a lower rise on the western side. Along the top of the eastern side a series of breastworks had been constructed by the bandits. In an apparent attempt to make the fortifications appear more daunting, yucca stalks were fashioned to look like rifle barrels and positioned in the breastworks. At about 1:00 pm, the Wham party reached the site of the attack. Campbell, who was in the lead, was attempting to guide her horse around a boulder that was blocking the road as the rest of the party crested a hill. After the convoy halted, Sergeant Benjamin Brown led his men forward to try to move the obstacle out of the road while Corporal Isaiah Mays took a position at the rear of the convoy. As they approached the rock, the group saw evidence that the rock had been deliberately placed in the road. As they then looked up to see where the rock had come from, the soldiers saw two men stand up from a breastwork situated above them. Neither of the two men wore masks and the soldiers later identified their attackers as Wilfred T. Webb and Mark E. Cunningham. Local folklore suggests some bandits may have donned disguises, with one attacker dressed to resemble a local figure named William Ellison "Cyclone Bill" Beck. Cunningham was armed with a rifle. Webb brandished a pair of revolvers and yelled, "Get out, you black sons of bitches!" The bandits then began to fire toward the convoy.