Fertility and marriage are inextricably linked in sub-Saharan Africa, but recent changes, such as the rise in non-marital fertility, signals a weakening link, and the second demographic transition offers some explanations. Non-marital fertility comes with disadvantages, but it has not been adequately studied in Nigeria. Hence, this study examined the levels, patterns, and correlates of non-marital fertility, and offers implications for interventions and future research. Using data from the Nigeria Demographic and Survey 2008–2018, with a pooled weighted sample size of 11,925 unmarried women, percentage distribution was employed and a two-part model for count data was fitted, with the result showing that the level of non-marital fertility is 29%, and it is common among younger, rural dwelling, and uneducated unmarried women. The correlates of non-marital fertility include age, region of residence, level of education, religion, household wealth index, relationship status, ethnicity, work status, and age at sexual debut. Interventions to arrest rise of non-martial fertility due to its obvious disadvantages, should strengthen sexual and reproductive health programs for unmarried rural-dwelling young women, and revitalize welfare efforts for children born outside wedlock, for poor women, while future research should explore an in-depth understanding of non-marital births.