DC-segmented power systems consist of ac subsystems connected to each other only by dc lines. DC-segmentation improves the transient stability and transmission capacity of power systems. Centralized control of large power systems entails burdensome communication requirements which are expensive and introduce delays. Decentralized control circumvents these problems, but finding decentralized controllers is intractable for general systems. Poset-causal systems are a special class for which the optimal neighbor-to-neighbor decentralized controller is easy to obtain. In this paper, we show that dc-segmented systems are poset-causal. We observe in numerical examples that the optimal decentralized controller attains almost the same performance as the optimal centralized controller.