Although portable heart rate (HR) monitors are commonly used to assess energy expenditure, little is known about their suitability for preschoolers. To validate the Polar Vantage XL monitor (XL), the HRs of twenty-seven 3- to 5-year-old girls and boys were measured using the XL and ECG simultaneously. During rest, values for both methods were virtually identical (ECG = 97.3 ± 7.5, XL = 97.9 ±7.2, r = .99 for lying, and ECG = 111.1 ± 16.5, XL = 110.4± 16.3 for sitting). XL nonsignificantly underestimated HR during 1–2 min of non-steady-state cycling (ECG = 142.7 ± 11.0, XL = 140.2 ± 11.5, r = .93) and significantly overestimated it during recovery (ECG = 112.4 ± 12.8, XL = 118.0 ± 12.3, r = .92). To assess social acceptability of wearing the XL, twenty-three 3- to 5-year-old girls and boys were observed twice for 60–90 min. Approximately 90% of the time, their responses were rated as enthusiastic/positive or agreed. Only 3–5% of children refused to wear the XL. In conclusion, the XL is highly valid and socially acceptable, when used with 3- to 5-year-old children.