Design of a stable controller and observer for web tension regulation in an accumulator, in web processing lines, is considered in this paper. A simplified average dynamic model that includes accumulator carriage dynamics, average web tension dynamics in accumulator web spans, and the process and exit side driven roller dynamics is considered in the study. A feedback controller together with an observer for average web tension is proposed. It is shown that the proposed feedback controller and observer results in a stable closed-loop system. Simulation results on an industrial continuous web processing line are given and discussed for the proposed controller/observer and compared with a currently used industrial controller. Applications of this research can be found in web process lines where accumulators are used to store/release material to ensure continuous operation.