Despite the increase in research on coparenting, few studies have focused on non-North American or non-European families, which has hindered practice and policy targeting diverse countries. The authors used a qualitative, longitudinal, multiple case study to investigate coparenting agreement/disagreement and support/undermining, defined by Feinberg's model, in a sample of South-Brazilian families across the Transition to Parenthood (TtoP). Twelve first-time mothers and fathers (six nuclear families) of children who attended different childcare arrangements (i.e., maternal care, nanny care, and daycare center) participated in individual, semi-structured, face-to-face interviews at 6, 12, and 18 months postpartum. Deductive thematic analysis was adopted to explore and interpret the data. Similarities and singularities between families were found. Overall, agreement remained relatively stable during the first year, whereas disagreements concerning discipline demanded more parental negotiation as infants advanced toward toddlerhood. Support and undermining coexisted in the same families, although mothers and fathers expressed undermining differently. Our findings also revealed how Brazilian sociocultural aspects linked to the upbringing in the family of origin, gender role expectations, labor and financial spheres, as well as childcare arrangements, may have shaped the coparenting dynamics of the participants. This study contributes to the literature by shedding light on coparenting in South-Brazilian families. Our findings offer support to two key themes aligned with Feinberg's model of coparenting, that is agreement/disagreement and support/undermining, further reinforcing the importance of understanding coparenting in light of the families' ecological context, without disregarding singularities that mark each coparental relationship. (PsycInfo Database Record (c) 2021 APA, all rights reserved).