Full text article. Also available at https://doi.org/10.1371/journal.pone.0216520Introduction Despite the benefits of early antenatal care visits for early prevention, detection, and treatment of potential complications in pregnancy, a high level of provider adherence to first-visit antenatal care standards is needed. However, little information is available regarding provider adherence to antenatal care in Tanzania. This study was performed to assess provider adherence to first-visit antenatal care standards and to apply stratified analysis to identify associated factors in Tanzania. Methods Data from the 2014–2015 Tanzania Service Provision Assessment Survey were used in this study. Provider adherence to first-visit antenatal care standards was measured using 10 domains: client history; aspects of prior pregnancies; danger signs of the current pregnancy; physical examination; routine tests; HIV testing and counseling; maintaining a healthy pregnancy; iron/folate supplements; tetanus toxoid vaccination, and preparation for delivery. A composite score was then created in which the highest quantile (corresponding to �60.5%) considered to provider adhering to first-visit antenatal care standards. Initially, a series of unadjusted logistic regression analyses according to the type of facility and managing authority were performed separately at each level (i.e., facility, provider, and client). Thereafter, all variables with P < 0.2 were fitted into the respective stratified multivariable logistic regression analysis using a 5% significance level. Results A total of 1756 first-visit antenatal care consultations performed by 822 providers in 648 health facilities were analyzed. The overall median [Interquartile range, IQR] adherence to first-visit antenatal care was relatively low at 47.1% [35.7%–60.5%]. After adjusting for selected variables from each level in specific strata, at dispensary; female providers [AOR = 5.5; 95% CI, 1.8–16.4], at health centre; performance of quality assurance [AOR = 2.2; 95% CI, 1.3–3.9], at hospital; availability of routine tests [AOR = 2.5; 95% CI, 1.3–4.8] and basic medicine [AOR = 2.8; 95% CI, 1.4–5.7], at public facilities; availability of medicine [AOR = 1.8; 95% CI, 1.1–3.2] and receiving refresher training [AOR = 1.8; 95% CI, 1.1–3.1], and at private facility; receiving external fund from government [AOR = 3.0; 95% CI, 1.1–8.4] were significantly associated with better adherence to first-visit antenatal care standards. Conclusions The study highlighted the important factors, including the provision of refresher training, regular distribution of basic medicines, and diagnostics equipment which may influence provider adherence to first-visit ANC standards.