Background: Adjuvant chemotherapy can reduce recurrence rates by eradicating microscopic metastases which may persist after curative resection. However, the optimal time interval (TI) between the surgery and chemotherapy remains controversial. Methods: The data were obtained from the NHIS of Korea. We included patients who underwent gastrectomy for gastric cancer between 2013 and 2018. To determine the optimal cut-off point of TI, a restricted cubic spline cox regression model was established, and categorized the population into three groups based on TI: the early group (≤ 20 days), the reference group (21-34 days), and the late group (≥ 35 days) with the reference interval group having the lowest mortality and recurrence. Propensity score matching was performed for each group. The primary outcomes were disease-free survival (DFS) and overall survival (OS). Results: During the study period, 98,556 patients underwent surgery for gastric cancer. After excluding ineligible participants, 6,602 patients were included in the analysis. The median DFS and OS did not differ significantly between the early and reference groups (p=0.7258 and p=0.6056, respectively). In comparison between the late and reference groups, the median DFS was significantly lower in the late group (p=0.0079). 5-year DFS were 77.6% and 81.3% in the late and reference groups, respectively. Furthermore, the late group showed worse OS than the reference group (p=0.0336). OS at 5-year were 82.1% and 85.0% in the late and reference groups, respectively. In the multivariable analysis, DFS in the late group retained inferiority (aHR 1.138, 95% CI: 1.003-1.292, p=0.045). OS showed a worse trend without significance compared to the reference group (aHR 1.138, 95% CI: 0.984-1.317, p=0.0805). Conclusions: Adjuvant chemotherapy after gastrectomy in patients with gastric cancer should be initiated within five weeks of surgery. A delay of more than five weeks may have a detrimental effect on the subsequent disease course.