Background: Prior research with adults and children suggests that inhibitory control may have a role to play in learning counterintuitive fractions and decimals that are inconsistent with whole number knowledge. However, there is little research to date with primary school-aged children at the early stages of fraction and decimal instruction that addresses this relationship. Understanding this association has the potential to inform instructional practices concerning the learning of counterintuitive maths concepts. Aim: This study examined the relationship between inhibitory control and counterintuitive fractions and decimals in the presence of varying cognitive load in 8- to 10-year-old children. Methods: Children aged 8-10 years (N = 95) completed a fraction and decimal magnitude comparison task with pairs that were either consistent (controls) or inconsistent (counterintuitive) with whole number magnitudes. Cognitive load was manipulated by presenting trials with simple integrated text (no additional load), with integrated text accompanied by supportive illustrations (low load), or with illustrations containing information that needed to be integrated to arrive at an answer (high load). Participants also completed measures of response and semantic inhibition. Results: Inhibitory control uniquely contributed to performance in counterintuitive fractions and decimals only under conditions of high cognitive load, where low semantic inhibition predicted longer response times. Conclusion: Our results indicate a more nuanced relation between inhibitory control and counterintuitive fractions and decimals than presumed by previous research. They suggest that the role of inhibitory control when reasoning about counterintuitive fractions and decimals is not constant, and it is only drawn on at high levels of cognitive load.