Background: Universal salt iodization (USI) is the most feasible and cost-effective, and equitable, approach to prevent iodine deficiency. Severe maternal iodine deficiency during pregnancy is associated with serious adverse gestational and birth outcomes. Objectives: The aim was to assess iodine status and identify independent factors associated with urinary iodine concentration (UIC) among women of reproductive age in Tanzania. Methods: This was a weighted, population-based, cross-sectional study in 2985 women of reproductive age (20-49 y) in Tanzania who participated in the Demographic and Health Surveys in 2015-2016 (DHS 2015-2016) and had measured UIC. Multivariable generalized linear regression was used to identify potential factors that were associated with UIC. Results: The median UICs among women consuming inadequately iodized salt (93.6 μg/L; 25th and 75th percentiles: 43.1, 197.9 μg/L) and women in the lowest socioeconomic status (92.3 μg/L; 45.6, 194.4 μg/L) were below the WHO-recommended ranges (≥150 μg/L for pregnant women and ≥100 μg/L for nonpregnant women). The results of multivariable models indicated that pregnant women had 1.21 μg/L lower UIC than nonpregnant women (β =-1.21; 95% CI:-3.42,-0.12), breastfeeding women had 1.02 μg/L lower UIC than nonbreastfeeding women (β =-1.02; 95% CI:-2.25,-0.27), and women with no education had a 1.88 μg/L lower UIC compared with those with secondary/highest education (β =-1.88; 95% CI:-4.58,-0.36). Women consuming inadequately iodized salt had 6.55 μg/L lower UIC than those consuming adequately iodized salt (β =-6.55; 95% CI:-9.24,-4.33). The median UIC varied substantially across geographic zones, ranging from 83.2 μg/L (45.9, 165.3) in the Western region to 347.8 μg/L (185.0, 479.8) in the Eastern region. Conclusions: Our findings indicated a great heterogeneity in median UIC across regions of Tanzania among women of reproductive age. Poverty, consuming inadequately iodized salt, and lack of education appeared to be the driving factors for lower UIC in Tanzania.