This paper demonstrates how object-oriented programming (OOP) can be used to create a program for topology optimization of steel trusses that employs multi-population particle swarm optimization (PSO). The OOP technique considers programs as communications and actions between entities called objects. Each object is a collection of data and their behavior. PSO mimics the swarm behavior of real animals. Each particle represents an animal and a population of particles therefore represents a population of animals. If OOP is used to implement PSO, each entity in PSO can be implemented as an object without any alteration to its physical meaning. In this study, OOP is used to develop a program for multi-population PSO for topology optimization of steel trusses. The development is natural and simple. The treatment of multiple populations of particles and the interaction between the populations can be done by simply creating objects, with appropriate data and behavior, to represent these populations. The OOP technique allows the extension of the ordinary PSO with one population to PSO with multiple populations to be done effortlessly.