Purpose: To explore the differences in apparent diffusion coefficient (ADC) values based on the primary tumor and sentinel lymph node (SLN) for predicting N stages of gastric cancer (GC). Methods: One hundred and sixty histopathologically confirmed GC patients between April 2021 and October 2024 were prospectively recruited. Preoperative DW-MRI was performed, and ADC values from primary tumors (ADC _T ) and SLNs (ADC _LN ), along with their relative ratios (rADC _T , rADC _LN ), were measured. Differences in these parameters across N stages were analyzed using the Kruskal-Wallis test. Receiver operating characteristic analysis was used to evaluate their diagnostic performances for predicting N0 vs. N1–3 stages, N0 + 1 vs. N2 + 3 stages, and N0 + 1 + 2 vs. N3 stages. Results: Significant differences were observed in ADC _T , rADC _T , ADC _LN , and rADC _LN values across N stages (all p < 0.001). The AUC values of ADC _T , rADC _T , ADC _LN , and rADC _LN for predicting N0 vs. N1 + 2 + 3 stages were 0.753, 0.727, 0.782, 0.792, respectively. The AUC values of ADC _T , rADC _T , ADC _LN , and rADC _LN for predicting N0 + 1 vs. N2 + 3 stages were 0.776, 0.767, 0.844, 0.837, respectively. The AUC values of ADC _T , rADC _T , ADC _LN , and rADC _LN for predicting N0 + 1 + 2 vs. N3 stages were 0.797, 0.792, 0.857, 0.848, respectively. Conclusions: Both primary tumor- and SLN-derived ADC values can effectively differentiate N stages among patients with GC. SLN-based ADC parameters exhibit superior diagnostic performance compared to primary tumor-based measurements in stratifying N-stage progression.