Triclosan, an antimicrobial chemical that was widely used in consumer products, may increase risk of allergic diseases in children, but prospective studies are needed to clarify the association. To elucidate the associations of time-varying urinary triclosan concentrations with eczema, allergic rhinitis, and wheeze. In the HOME Study, a prospective pregnancy and birth cohort, we quantified urinary triclosan concentrations in mother-child pairs up to ten times between 16 weeks' gestation and age 12 years. Caregivers reported eczema, allergic rhinitis, and wheeze symptoms biannually until children were aged 6 years and again when they were aged 8- and 12 years. We used generalized estimating equations to estimate the covariate-adjusted association of gestational and childhood triclosan concentrations with the risk of reporting eczema, allergic rhinitis, or wheezing symptoms. Three hundred forty-seven mother-child dyads contributed >3,000 visits to the analysis of gestational exposures and >2,600 visits to childhood analyses. Each 2-fold higher childhood triclosan concentration was associated with a 1.23 (95% CI: 1.04, 1.46) and 1.12 (95% CI: 1.01, 1.25) times higher risk of reporting eczema and allergic rhinitis, respectively, but not wheezing (RR=0.98 [95% CI: 0.82, 1.16]). We did not observe modification by child sex. Associations of gestational triclosan with eczema, allergic rhinitis, or wheezing symptoms were null in the full sample. Child sex modified the association of gestational triclosan with allergic rhinitis and wheezing (p-interactionAllergy: 0.02; p-interactionWheezing:0.10), with 1.09 (95% CI: 1.00, 1.19) and 0.91 (95% CI: 0.83, 1.00) times the risk of allergic rhinitis symptoms among males and females respectively. Childhood urinary triclosan concentrations were associated with caregiver reported eczema, and more weakly with allergic rhinitis. Associations of gestational triclosan with allergic outcomes differed by child sex, suggesting heightened susceptibility to triclosan among males. https://doi.org/10.1289/EHP16710.