Humans are curious. Especially children are known for their drive to explore and learn, which is crucial for developing in and navigating through our complex world. Naturally, some children may be more curious than others, leading to differences in how they structure their own learning experiences, subsequently impacting their developmental trajectories. However, there is a gap in the research field for a reliable measure of such differences early in development. Across three studies, we present the development and assessment of the Infant and Toddler Curiosity Questionnaire (ITCQ), the first caregiver report measure to fill this gap. Items cover observable exploration behaviors in 5- to 24-month-olds to capture general tendencies of their desire to actively explore their immediate surroundings and are evaluated on a 7-point Likert-scale. Exploratory factor analyses and structural equation modeling on a sample of N = 370 UK caregivers led to the final selection of 23 items and provided evidence that the scale allows the reliable computation of an overall curiosity score, with three emergent subscales (Sensory, Investigative, and Interactive) explaining additional variance in the data. Furthermore, the scale had good test-retest reliability after 7–14 days (N = 67) and related to the child's temperament (N = 75; positively with surgency and effortful control, negatively with negative affect) offering evidence of its validity as a trait measure. Together, these results support the scale's reliability and validity, showcasing the ITCQ as a powerful tool for developmental research.