Background/Objectives: Postoperative early hypocalcemia (PEH) is a key postoperative issue after parathyroidectomy in primary hyperparathyroidism. It often leads to long-lasting hypocalcemia, requiring more calcium and active vitamin D supplements. This study aimed to determine whether the extent of intraoperative parathyroid hormone (PTH) decline, measured 15 min after parathyroid tumor excision, could serve as a reliable intraoperative rule-out marker for PEH. Methods: We conducted a retrospective review of 88 adult patients who underwent surgical intervention for a solitary parathyroid tumor at a single institution. Postoperative early hypocalcemia (PEH) was defined as a total serum calcium level <8.5 mg/dL within the postoperative 6th hour or on postoperative day 1, requiring clinical calcium supplementation (oral and/or intravenous), with active vitamin D when appropriate. The percentage decrease in PTH at 15 min post-excision was calculated using morning-of-surgery preoperative PTH values alongside the 15-min post-excision levels. Additional variables assessed included preoperative alkaline phosphatase (ALP), parathyroid tumor weight, and serum concentrations of calcium, phosphate, magnesium, and 25-hydroxyvitamin D. Predictive factors were identified by logistic regression, and the diagnostic accuracy of the 15-min PTH decline was evaluated using receiver operating characteristic (ROC) curve analysis, optimizing cutoff selection with Youden’s index. Odds ratios were standardized per 10-unit increments for ALP and parathyroid tumor weight for interpretability. Results: Of the studied cohort, 10 patients (11.4%) developed PEH. The intraoperative 15-min PTH decline was notably greater in those who developed PEH compared to those who did not (81.2 ± 4.4% vs. 69.9 ± 8.3%; p < 0.001). Univariate logistic regression showed a significant association between the 15-min PTH decline and PEH (OR 1.22 per 1% increment; 95% CI 1.08–1.38). That said, when we added ALP and parathyroid tumor weight to the multivariate models, PTH decline no longer predicted independently. In contrast, ALP (OR 3.11 per 10 U/L; 95% CI 1.34–7.93; p = 0.011) and parathyroid tumor weight (OR 1.22 per 10 mg; 95% CI 1.10–1.48; p = 0.004) stayed significant. Thus, the incremental prognostic contribution of the 15-min PTH decline beyond ALP and parathyroid tumor weight appears limited. The ROC curve for the 15-min PTH decline produced an AUC of 0.883, with an optimal cutoff of 75% providing 100% sensitivity and 74.4% specificity. No patients with a PTH decline below 75% developed PEH. Conclusions: Preoperative ALP and parathyroid tumor weight showed the strongest independent associations with PEH following parathyroid tumor surgery. An intraoperative PTH decline of less than 75% at 15 min may serve as a practical rule-out tool for PEH, although further validation in larger patient populations is warranted.