While thinking in constraints is a key element of frugal innovation, it remains underexplored in the context of developing frugal innovations. Based on insights from the literature and two cases from the medical device industry, this article introduces the concept of constraint-based thinking to drive the frugal product development process based on constraints. This approach is operationalized through a systematic and iterative process to investigate the visible and underlying symptoms of different types of constraints, which are then mapped to specific product features and requirements. It consists of a problem space and a solution space, which are based on the idea that constraints are opportunities to develop innovative solutions. The main components of the constraint-based thinking approach are as follows: first, the identification of constraints or existing limitations, hindrances behind the current status quo that need to be changed; second, a root-cause analysis to understand the underlying causes for each of the identified constraints; third, a mapping of each of those causes to specific product characteristics or requirements; and, fourth, development of a minimal viable product or a prototype. This approach can be used both by companies to develop frugal innovations and by researchers to further develop the link between constraints and new products and services.