The aim of this paper is to design and implement a single-legged vertical hopping robot with a pneumatic cylinder. The energy analysis is used to determine the hopping cycle and height for the vertical hopping robot. The mathematical model of the system is derived. Second order sliding mode control is used to design control laws to control the mass flow rate for the upper chamber and lower chamber of the cylinder. The numerical simulations are conducted. In experiments, proportional valves are used for control actuation. For feedback control, the strokes of the cylinder rod and the hopping height of the cylinder are measured by a potentiometer and a magnetic encoder, respectively. The control schemes are implemented on a digital signal processor. From simulation and experimental results, it is shown that the designed sliding mode controllers can achieve effective hopping control.