Testes and ovaries undergo sex-specific morphogenetic changes and adopt strikingly different morphologies, despite the fact that both arise from a common precursor, the bipotential gonad. Previous studies showed that recruitment of vasculature is critical for testis patterning. However, vasculature is not recruited into the early ovary. Peripheral innervation is involved in patterning development of many organs but has been given little attention in gonad development. In this study, we show that while innervation in the male reproductive complex is restricted to the epididymis and vas deferens and never invades the interior of the testis, neural crest-derived innervation invades the interior of the ovary around E16.5. Individual neural crest cells colonize the ovary, differentiate into neurons and glia, and form a dense neural network within the ovarian medulla. Using a sex-reversing mutant mouse line, we show that innervation is specific to ovary development, is not dependent on the genetic sex of gonadal or neural crest cells, and may be blocked by repressive guidance signals elevated in the male pathway. This study reveals another aspect of sexually dimorphic gonad development, establishes a precise timeline and structure of ovarian innervation, and raises many questions for future research. © 2019 National Academy of Sciences. All Rights Reserved.