Using longitudinal data from the Nairobi Urban and Demographic Surveillance System (NUHDSS), we examined the seasonal pattern of pneumonia mortality among under-five children living in Nairobi’s slums. We included 17,787 under-five children resident in the NUHDSS from January 1, 2003 to December 31, 2005 in the analysis. Four hundred thirty-six deaths were observed and cause of death was ascertained by verbal autopsy for 377 of these deaths. Using Poisson regression, we modeled the quarterly mortality risk for pneumonia. The overall person-years (PYs) were 21,804 giving a mortality rate of 20.1 per 1,000 PYs in the study population. Pneumonia was the leading cause of death contributing 25.7% of the total deaths. Pneumonia mortality was highest in the second quarter (risk ratio [RR] = 2.3, confidence interval [CI]: 1.2–4.2 compared with the fourth quarter). The study provides evidence that pneumonia-related mortality among under-fives in Nairobi’s slums is higher from April to June corresponding to the rainy season and the beginning of the cold season.