The majority of studies of preschool-aged children’s self-regulation presume that their higher levels of self-regulation are concurrently and prospectively associated with fewer externalizing and internalizing problems. This assumes their relations are only linear in form and negative, but studies with community samples of mostly non-Hispanic White children have found curvilinear or positive relations between self-regulation and socioemotional problems in early childhood. This cross-sectional study tests linear and quadratic relations between children’s behavioral battery assessed effortful control and parent rated externalizing and internalizing problems, and whether their functional forms differ across racial-ethnic groups in a diverse sample of 2.5- to 3.5-years-olds (N = 72) from highly educated two-parent households. Child effortful control was negatively related to externalizing, quadratically related to internalizing (albeit marginally), and an interaction between effortful control and race-ethnicity indicated opposite linear relations between effortful control and internalizing problems for different racial-ethnic groups. By integrating tests of curvilinearity and interactions, this study builds on theoretical and empirical work indicating complex relations between the development of self-regulation and psychopathology.