Several epidemiological studies have shown the relationship between hospital admissions associated with asthma, meteorological variables, and exposure to outdoor air pollution. In this study, we propose to evaluate the effects of environmental variables on asthma admissions in the Lisbon Metropolitan Area, Portugal. A quasi-Poisson generalized additive model combined with a distributed lag non-linear model (DLNM) was applied to estimate the delayed effects of environmental variables on childhood asthma admissions from 2009 to 2015. Subgroup analyses by sex and age were performed. We found that an increase of 1 μg/m3 of PM10 was associated with an increased risk of asthma-related hospital admission, on the order of 2% (RR = 1.02; CI 95% 1.01–1.03). There was an increase of 4% associated with a rainfall (PP) increase of 1 mm (RR = 1.04; 95% CI 1.02–1.11). Among male children, the increase of PM10 concentration is associated with a 2% increase in the risk of hospital admission, while PP is associated with a 5% increase. PM10, Tmean, and Tmax are significantly associated at lags 0–3 months, for age group 5–9 years, with a cumulative effect of RR = 1.03; 95% CI (1.01–1.05); RR = 1.54; 95% CI (1.19–1.99); and RR = 0.67; 95% CI (0.54–0.84) at lag 0, respectively. For age group 10–14 years, only relative humidity (RH) was significantly associated with hospital admissions; an increased risk was with higher relative humidity and short term (lag 0). Our results provide evidence of the adverse effects of PM10, PP, and RH on childhood asthma. These variables are good predictors of asthma. The risk of hospital admission with a primary diagnosis of asthma is more frequent during the autumn, when the school year begins. These findings will be important for health authorities as they provide epidemiological evidence for developing prevention strategies to reduce the risk of childhood asthma hospitalizations.