Background: We present an analysis of the short-term associations between exposure to nitrogen dioxide (NO₂) in ambient air and mortality by cause, age group, and season across 18 study areas in metropolitan France. Methods: Associations between NO₂ and daily mortality were examined in each study area using a generalized additive model with a Poisson distribution; the resulting risk estimates were then combined in a meta-analysis. The percentage increase in mortality associated with a 10 µg/m³ increase in NO₂ on preceding days was estimated for the 2010–2014 period and by season. Results: A 10 µg/m³ increase in NO₂ on the current day and the previous day (lag 0–1) results in a 0.75% increase in non-accidental mortality (95% CI: [0.4–1.10]). The effects of NO₂ are greater among people aged 75 and older. During the warm season, a 10 µg/m³ increase in NO₂ is associated with a 3.07% increase in non-accidental mortality the following day (95% CI: [1.97–4.18]) among people aged 75 and older. These associations remain stable after adjustment for PM10. Conclusion: Our results confirm that exposure to NO2, as a marker of traffic-related pollution, is associated in the short term with mortality, even at concentrations—on an annual average—that comply with European regulations and the WHO guideline value (40 µg/m³). They also provide exposure-risk relationships that can be used to conduct quantitative assessments of the health impact of air pollution.