<p>Children incorporate items found in their environment into their play, transforming everyday objects and materials into an opportunity for exploration. Termed loose parts, these versatile, natural, or manufactured materials (e.g., cardboard, pipes, buttons, sticks) are widely recommended for supporting children’s early STEM learning. Limited empirical work has documented children’s indoor STEM behaviours with loose parts. Using a within-subjects experimental design, we examined children’s early STEM behaviours and engagement (<i>N</i> = 60; 32 females, 28 males; <i>Mage</i> = 58.6 months, <i>SD</i> = 10.9) during unstructured solitary play with loose parts and toys that have limited function and affordance (e.g., toy percussion instruments; control). We conducted observations of children’s STEM behaviours. Children’s cognitive functioning, executive function, and home learning environment were also assessed via standardized measures and parent reports. Children demonstrated significantly more STEM behaviours with loose parts than in the control condition. There was no credible evidence that these behaviours differed by sex. Cognitive functioning predicted STEM Engagement Score with loose parts, with children’s verbal comprehension being the strongest predictor in the control condition. Children’s executive function and parents’ attitudes regarding play and engagement in play activities at home predicted constructing structures, which were the most common STEM behaviours. This study thus demonstrates that loose parts may offer a powerful opportunity for STEM-related early learning; however, children’s cognitive capacities and home experiences should be considered, rather than assuming uniform benefits.</p>