There is a need for a modified Global Physical Activity Questionnaire (GPAQ) to estimate physical activity levels more accurately in populations. We aimed to develop a modified GPAQ and examine its construct validity among adults in Kerala state, India. We incorporated locality-specific, metabolic equivalent task (MET)-based activities into the original GPAQ and administered this modified GPAQ among randomly selected 451 individuals (age ≥20 years) residing in the Trivandrum district of Kerala. Construct validity was assessed using Generalized Linear Models by examining the association of total moderate-to-vigorous physical activity (MVPA) MET-minutes per week estimated by the modified GPAQ with clinical measures. The mean age of participants was 45.4 (SD: 14.1) years and 52.6% were female. Increasing total MVPA MET-minutes per week was associated with decreasing weight (β=-0.011 kg, 95% CI: -0.020, -0.002), waist circumference (β=-0.013 cm, 95% CI: -0.023, -0.004), and systolic blood pressure (β=-0.009 mmHg, 95% CI: -0.015, -0.002), independent of age, sex, education, occupation, current smoking, current alcohol use, and fruit and vegetable intake. The validity coefficients and associations between total MVPA MET-minutes per week and theoretical constructs of physical activity were in agreement with those predicted, providing evidence of construct validity for the modified GPAQ.