This article reports a multimethod study of product innovation processes in small manufacturing firms. Prior studies found that small firms do not deploy the formalized processes identified as best practice for the management of new product development (NPD) in large firms. To explicate small firms' product innovation, this study uses effectuation theory, which emerged from entrepreneurship research. Effectuation theory discerns two logics of decision‐making: causation, assuming that means are selected to attain goals; and effectuation, assuming that goals are created based upon available means. The study used a process research approach, investigating product innovation trajectories in five small firms across 352 total events. Quantitative analyses revealed early effectuation logic, which increasingly turned toward causation logic over time. Further qualitative analyses confirmed the use of both logics, with effectual logic rendering product innovation resource‐driven, stepwise, and open‐ended, and with causal logic used especially in later stages to set objectives and to plan activities and invest resources to attain objectives. Because the application of effectuation logic differentiates the small firm approaches from mainstream NPD best practices, this study examined how small firms' product innovation processes deployed effectuation logic in further detail. The small firms: (1) made creative use of existing resources; (2) scoped innovations to be realizable with available resources; (3) used external resources whenever and wherever these became available; (4) prioritized existing business over product innovation projects; (5) used loose project planning; (6) worked in steps toward tangible outcomes; (7) iterated the generation, selection, and modification of goals and ideas; and (8) relied on their own customer knowledge and market probing, rather than early market research. Using effectuation theory thus helps us understand how small firm product innovation both resembles and differs from NPD best practices observed in larger firms. Because the combination of effectual and causal principles leverages small firm characteristics and resources, this article concludes that product innovation research should more explicitly differentiate between firms of different sizes, rather than prescribing large firm best practices to small firms.