The presence or absence of pulsus paradoxus (PP), defined as an inspiratory decrease greater than 10 mmHg in systolic blood pressure, can have significant diagnostic and therapeutic implications for many clinical conditions including acute asthma, pericardial tamponade, heart failure, hypovolemia, shock states, and the like. However, PP may be difficult to measure in children. Indwelling arterial catheters facilitate the measurement of PP, but this invasive technique generally is reserved for critically ill patients. This study aimed to assess the use of the pulse oximetry plethysmographic waveform (POPW) for the detection of PP in pediatric patients after cardiac surgery. The study enrolled 40 pediatric patients 18 years of age and younger who had invasive blood pressure monitoring with an intraarterial cannula. Systolic pressure variability (SPV) and changes in POPW amplitude (ΔPOPW%), calculated using five consecutive snapshots from every patient’s monitor, were compared using linear regression, Pearson product–moment correlation, the Spearman rank method, and receiver operating characteristic (ROC) curve analysis. A strong correlation existed between respiratory SPV and ΔPOPW% for the detection of PP (r = 0.682; p < 0.0001). A respiratory variation in ΔPOPW% exceeding 25.44% (about one-fourth the amplitude of the tallest POP waveform) allowed detection of PP with a sensitivity of 86.7% and a specificity of 88%. Pulse oximetry is a readily available and easily performed noninvasive means for detecting PP in children.