Heat stress is a major challenge facing the dairy industry, negatively impacting cow health, milk production, and reproductive performance. Early detection of heat stress is critical for implementing timely mitigation strategies. This study evaluated the use of body activity and rumination time measured by HR-Tag sensors for early detection of heat stress in lactating Holstein cows. Cows (n=100) were housed in a free-stall barn and monitored during summer months. HR-Tag sensors measured body activity (bouts/hr) and rumination time (min/hr). Ambient temperature and humidity were recorded to calculate temperature-humidity index (THI). Mixed linear models analyzed relationships between THI and cow activity and rumination. As the temperature-humidity index (THI) increased, rumination time decreased significantly (r = -0.56, P < 0.001), while activity levels increased (r = 0.43, P < 0.05). Compared to normal conditions (THI < 68), cows under mild heat stress (68 ≤ THI < 72) spent an average of 31.59 minutes less per day ruminating (6.16% reduction) and exhibited a 5.1% increase in activity level. Under moderate heat stress (72 ≤ THI < 80), rumination time decreased by 59.55 minutes (11.42% reduction), and activity levels increased by 14.31%. Severe heat stress (THI ≥ 80) had the most pronounced effects, with cows spending 104.26 minutes less per day ruminating (20.35% reduction) and exhibiting a 24.26% increase in activity level compared to normal conditions. Automated monitoring of body activity and rumination time, especially in high producing cows, allows early detection of heat stress before reductions in milk yield and health issues arise. This enables producers to implement cooling and management changes proactively to mitigate negative impacts of heat stress.