This paper presents a new distributed observer for interconnected multi-rate systems. The developed observer belongs to the family of set-membership estimators, and the use of zonotopes is proposed to mathematically describe the sets, a choice motivated by the available mathematical background in operations such as intersections, combinations and linear manipulations. The main features of the proposed distributed observer are (a) the actual state of any subsystem always belongs to the computed sets; (b) the volume of these sets is minimized in real time; (c) under equivalent assumptions, the performance of the observer approaches to that of an analogous distributed Kalman filter. (C) 2017 Elsevier Ltd. All rights reserved.