This paper studies the problem of using a sampled-data output feedback controller to globally stabilize a class of nonlinear systems with uncertain measurement and control gains. A reduced-order observer and a linear output control law, both in the sampled-data form, are designed without the precise knowledge of the measurement and control gains except for their bounds. The observer gains are chosen recursively in a delicate manner by utilizing the output feedback domination approach. The allowable sampling period is determined by estimating and restraining the growth of the system states under a zero-order-hold input with the help of the Gronwall–Bellman Inequality. A DC–DC buck power converter as a real-life example will be shown by numerical simulations to demonstrate the effectiveness of the proposed control method.