Background: Pregnant women are vulnerable to iron deficiency due to the fact that more iron is needed primarily to supply the growing fetus and placenta and to increase the maternal red cell mass. Little is known on the factors associated with uptake of iron supplement during pregnancy.Methods: The study used data from the 2015 to 2016 Tanzania Demographic and Health Survey and Malaria Indicators Survey. A total of 6,924 women of active reproductive age from 15 to 49 were included in the analysis. Both univariate and multiple regression analyses were used to determine factors associated with uptake of iron supplement during pregnancy.Results: Majority of the interviewed women 5,648 (81.6%) always took iron supplement during pregnancy, while a total of 1,276 (18.4%) women never took iron supplement during pregnancy. After controlling for confounders, the predictors for uptake of iron supplement during pregnancy were early antenatal booking (adjusted odds ratio, AOR = 1.603 at 95% CI = 1.362–1.887, p < 0.001); rural residence (AOR = 0.711 at 95% CI = 0.159–0.526, p = 0.007); wealth index [rich (AOR = 1.188 at 95% CI = 0.986–1.432, p = 0.07)]—poor was the reference population; level of education [primary education (AOR = 1.187 at 95% CI = 1.013–1.391, p = 0.034)]—no formal education was the reference population; parity [para 2 to 4 (AOR = 0.807 at 95% CI = 0.668–0.974, p = 0.026), para 5 and above (AOR = 0.75 at 95% CI = 0.592–0.95, p = 0.017)], para 1 was the reference population; zones [mainland rural (AOR = 0.593 at 95% CI = 0.389–0.905, p = 0.015) and Unguja Island AOR = 0.63 at 95% CI = 0.431–0.92, p = 0.017]—mainland urban was the reference population; and current working status [working (AOR = 0.807 at 95% CI = 0.687–0.949, p = 0.009)].Conclusion: The study revealed that, despite free access to iron supplement during pregnancy, there are women who fail to access the supplement at least once throughout the pregnancy. The likelihood to fail to access iron supplement during pregnancy was common among pregnant women who initiated antenatal visits late, were from poor families, had no formal education, reside in rural settings, had high parity, were from mainland rural, and were in working status. Interventional studies are recommended in order to come up with effective strategies to increase the uptake of iron supplement during pregnancy.