Aphid infestation and subsequent yield loss are a big problem for legume cultivation. Sowing time plays a critical role in influencing aphid management and seed yield in legume crops, as demonstrated in this study on BARI Khesari (Grasspea)-3 (Lathyrus sativus) conducted during two consecutive rabi season (2020-2021) at the Pulses Research Centre (PRC), Bangladesh Agricultural Research Institute (BARI), Ishwardi, Pabna. In both years, seven sowing dates were tested at weekly intervals, starting from 9th November 2020 to 21st December 2020 in the first year and 31st October 2021 to 12th December 2021 in the second year. The results consistently showed that late-sown crops, particularly those sown in December, experienced greater aphid infestation compared to crops sown in November. Aphids preferred the succulent growth stages of late-sown crops, leading to increased feeding and multiplication. In 2020, the highest seed yield (1511 kg/ha) was recorded from the crop sown on 16th November, followed by sowing on the 23rd November (1443 kg/ha), with yields decreasing in crops sown later. Similarly, in 2021, the highest yield (1040.28 kg/ha) was obtained from the crop sown on 14th November, while crops sown in December exhibited lower yields due to both delayed sowing and heavy aphid infestation. Twig infestation rates were significantly higher in late-sown crops, reaching 60.42% and 57.71% in crops sown on 5th and 12th December 2021, respectively. The findings from both years revealed the critical role of sowing time in managing aphid populations and enhancing seed yield. Mid-November was identified as the optimal sowing period to avoid severe aphid damage and achieve higher yields in BARI Khesari-3 cultivation.