Multi-terminal DC (MTDC) networks interconnect isolated systems, asynchronous areas, and renewable energy resources. However, ensuring stability and proper dynamic behavior of MTDC networks can be challenging. Power electronic converters offer increased control capabilities to achieve predictable and stable dynamics in the face of disturbances, faults, and sustained oscillations. The DC-side power system stabilizer (DC-PSS) is a supplementary controller that relies on optimal placement for maximized effectiveness. Observers can estimate the state of relevant system variables for the feedback controller without system-wide communication. This paper proposes an optimally-placed observer-based augmented DC-PSS for enhanced voltage stability in MTDC networks. The design procedure assumes no knowledge of the structure and parameters of the inner controllers of the terminal converters. We present a mathematical model and discuss the observer and control design procedures, and simulation results show the benefits and potential drawbacks of the proposed approach. Our observer-based DC-PSS relies on system-wide DC-bus voltage estimates from a single measurement, making it less vulnerable to communication delays and cyberattacks. Our work contributes to the field of supplementary controllers in MTDC networks and provides insights into future research directions toward stable and reliable DC power systems.