The present study investigated the association of accelerometer-determined moderate-to-vigorous physical activity (MVPA) with neck circumference (NC) in adolescents and adults from eight Latin American countries. The sample consisted of 2370 participants from the Latin American Study of Nutrition and Health, a multicenter cross-sectional nutrition and health surveillance study of a nationally representative sample from eight Latin American countries. An Actigraph GT3X+ accelerometer was used to monitor MVPA. Adolescents were categorized as NC > 34.25 cm in boys and NC > 31.25 cm in girls, and adults were categorized as NC > 39 cm in men and NC > 35 cm in women. The average time of MVPA was 34.88 min/day, ranging from 31.16 in Venezuela to 40.27 in Chile. Concerning NC, 37.0% of the sample was classified as having elevated NC. Chile was the country with the highest percentage of people with elevated NC (56.9%), and Colombia had the lowest percentage (24.8%). Overall, the MVPA (min/day) was associated with elevated NC (OR = 0.994, CI95% = 0.990–0.998). In Costa Rica and Peru, there were significant associations between MVPA and NC when analyzed by country. Further research is required to understand the differences between countries in the observed associations.