Background: We aimed to examine factors associated with prenatal syphilis, including prenatal care, and pregnancy outcomes of pregnant women with HIV in Brazil. Methods: Retrospective data were gathered from a national cohort of Brazilian women with HIV on antiretroviral therapy who became pregnant between January 2015 and May 2018. Prenatal syphilis was defined by clinical diagnoses with treatment or any positive syphilis laboratory result between 30 days before conception and pregnancy conclusion. Multivariable logistic regression models examined factors associated with prenatal syphilis risk and adverse pregnancy outcomes (including stillbirth, abortion, preterm delivery, small for gestational age, and congenital abnormalities). Receipt of recommended prenatal syphilis screening and adequacy of prenatal care were also evaluated. Findings: Among 2169 women, 166 (7.77% [95% CI: 6.5–8.8%]) had prenatal syphilis, of whom 151 (91%) had documented treatment. Prevalence of prenatal syphilis was higher among women of Black/Pardo/Indigenous race (13.7/7.7/8.3% vs. 5.8% in White women), those of younger age (median age 25.9 years vs. 27.6 in total cohort) and those with crack/cocaine use during/before pregnancy (20%). Of 1042/2169 women with prenatal care and screening data, 475 (46%) received inadequate prenatal care and only 301 (29%) received the recommended antenatal syphilis screening. Prenatal syphilis was not associated with adverse pregnancy outcomes (aOR 0.91 [0.64–1.30]). Interpretation: Prenatal syphilis was prevalent in this cohort of pregnant women with HIV. Prenatal syphilis was not associated with adverse pregnancy outcomes. Attention to syphilis prevention and treatment is especially needed in marginalised women.