Glaciers in Antarctica are flowing into the ocean and contributing to global sea level rise. Understanding how fast they move is important for assessing the stability of the Antarctic Ice Sheet. While most Antarctic glaciers tend to flow faster in summer and more slowly in winter, our study reveals that Byrd Glacier in East Antarctica shows a different seasonal pattern: its flow near the grounding line gradually slows down during the summer. Using satellite data and robust analysis methods, we identified this unique behavior and found a 70 km² active subglacial lake about 20 km upstream of the grounding line. We propose that the seasonal slowdown is linked to interactions between the ice, ocean, and especially the subglacial hydrological system. Our findings enhance understanding of ice–subglacial water–ocean interactions and underscore the need for comprehensive, year-round monitoring of glacier dynamics across the entire Antarctic Ice Sheet.