Background: Whether the dorsal striatum (DS) mediates cognitive control or cognitive effort per se in decision-making is unclear because as cognitive control requirements of a task intensify, cognitive effort requirements increase proportionately. We implemented a task that disentangled cognitive control and cognitive effort to specify the function DS mediates in decision-making. Methods: Sixteen healthy young adults completed a number Stroop task with simultaneous blood-oxygenation-level-dependent response (BOLD) measurement. Participants selected the physically larger number of a pair. Discriminating smaller physical size differences increases cognitive effort, but does not demand greater cognitive control. We also investigated the effect of interdimensional conflict between physical size and numerical magnitude. Selections in this incongruent case are more cognitively effortful and require greater cognitive control to suppress responding to the irrelevant dimension. Enhancing cognitive effort or cognitive control requirements increases response times and error rates. Results: Behavioural interference occurred for both conditions; however, DS BOLD signal only correlated with interference due to increased cognitive control requirements. DS was not preferentially activated for discriminations of smaller relative to larger physical size differences between number pairs, even when using liberal statistical criteria. Conclusions: Our findings support the increasingly accepted notion that DS mediates cognitive control specifically and does not index cognitive effort per se.