This paper considers the design and implementation of a low-cost and modular autonomous surface robot for inland water. The design process consists of three stages: Mechanical and electro-mechanical design, electrical and electronic design and software design. The mechanical design is based on a two-hull construction because of its low risk of capsizing in rough water. Off-the-shelf hulls and electric trolling motor are preferred to reduce the cost. The robot is steered by a rudder controlled by a servo motor. A Robot Operating System based software running on an on-board computer is developed to achieve autonomy. The robot’s status is monitored using the ground station software. The developed system was tested through a series of field experiments. The system is also compared with the existing designs. The robot’s available deck space and modular software architecture enable users to easily integrate various sensors and mechanical parts for a wide range of applications such as environmental monitoring, surveillance and patrolling.