CONTEXT: In rural South Africa, women often delay union formation until they are in their late 20s, though premarital first births are common. METHODS: Longitudinal data from the Agincourt Health and Socio-Demographic Surveillance System in rural South Africa were used to examine the relationship between premarital birth and union entry among 55,158 nonmigrant women aged 10–35 who took part in at least one annual census from 1993 to 2012. Discrete-time event history models were used to determine whether the likelihood of union formation differed between women who had had a premarital first birth and those who had not. Associations between single motherhood and union type (marriages or nonmarital partnerships) were identified using logistic regression. RESULTS: Forty-five percent of women had had a premarital first birth and 25% had entered a first union. Women who had had a premarital first birth were less likely than other women to have entered a first union (odds ratio, 0.6). Women who had had a premarital birth in the past year were more likely than those without a premarital birth to have entered a union (1.5), but women had reduced odds of union formation if they had had a birth 1–2 years earlier (0.9) or at least five years earlier (0.8). Unions formed within two years of a premarital birth had an elevated likelihood of being nonmarital partnerships (1.2–1.4). CONCLUSIONS: Single motherhood is common in the Agincourt HDSS, and women with a premarital first birth face challenges in establishing committed unions with partners.