Abstract: This paper deals with the problem of constructing a minimal-order state observer for Roesser-type two-dimensional systems. Sufficient conditions are derived for the existence of the observer and the design procedure is presented. It is shown that if a full-order observer can be constructed for a given two-dimensional system, the minimal-order observer can also be constructed. Finally an example shows that by the method presented in this paper, we can design a minimal-order observer for a two-dimensional system for which no state observer exists by the method of Hinamoto et al.