Abstract Background Financial access to family planning (FP) is essential to the health and well-being of women in Tanzania. Tanzanian policy dictates that FP methods and services obtained at public facilities are provided for free. However, public sector FP is no longer free when providers solicit informal payments. In this analysis, we investigate the prevalence and amount of informal payments for FP in Tanzania. Methods We used data from the 2015–2016 Tanzania Demographic and Health Survey to investigate whether informal payments for FP had been effectively eliminated by this policy. Results We found that, at public sector facilities, the majority (84.6%) of women received their current FP method for free (95% confidence interval (CI): 81.9, 87.3), but this proportion varied meaningfully by facility and method type. Injectable contraception was the most commonly used method by women in the lowest wealth quintiles and was most frequently sought by these women from a government dispensary. One in four women (25.8%) seeking injectable contraception from government dispensaries reported paying a fee (95% CI: 19.5, 32.1). Among injectable users who reported payment for their current method, the mean cost at public sector facilities was 1420 Tanzanian Shillings (TSh) and the mean cost at private sector facilities was TSh 1930 (approximately 0.61 United States Dollars (USD) and 0.83 USD, respectively). Among implant users who reported payment for their current method, the mean cost at public sector facilities was TSh 4127 and the mean cost at private sector facilities was TSh 6194 (approximately 1.78 USD and 2.68 USD, respectively). Conclusion These findings suggest that the majority of women visiting public facilities in Tanzania did not pay informal payments for FP methods or services; however, informal payments at public facilities did occur, varying by facility and method type. Adherence to existing policies mandating free FP methods and services at public facilities, especially government dispensaries, is critical for ensuring contraceptive access among the most economically vulnerable women.