Tobacco use is a major risk factor for tuberculosis (TB). Secondhand smoke (SHS) is also a risk factor for TB and to a lesser extent, Mycobacterium tuberculosis infection without disease. We investigated the added risk of M. tuberculosis infection due to SHS exposure in childhood contacts of TB cases in The Gambia. Participants were childhood household contacts aged ≤ 14 years of newly diagnosed pulmonary TB (PTB) cases. The intensity of exposure to the case was categorized according to whether contacts slept in the same room, same house, or a different house as the case. Contacts were tested with an enzyme-linked immunospot interferon gamma release assay. In multivariate regression models, M. tuberculosis infection was associated with increasing exposure to a case (odds ratios [OR]: 3.9, 95% confidence interval [CI]: 2.11-71.4, P < 0.001]) and with male gender (OR: 1.5 [95% CI: 1.12-2.11], P = 0.008). Tobacco use caused a 3-fold increase in the odds of M. tuberculosis infection in children who slept closest to a case who smoked within the same home compared with a nonsmoking case (OR: 8.0 [95% CI: 2.74-23.29] versus 2.4 [95% CI: 1.17-4.92], P < 0.001). SHS exposure as an effect modifier appears to greatly increase the risk of M. tuberculosis infection in children exposed to PTB cases. Smoking cessation campaigns may be important for reducing transmission of M. tuberculosis to children within households.