Ovary-specific acidic protein (OSAP) is an important reproduction-related gene. In this study, we cloned the full-length cDNA sequence of porcine OSAP gene through the rapid amplification of cDNA ends method. The porcine OSAP gene encodes a protein of 238 amino acids which shares high homology with the OSAP of five species: rhesus monkey (72 %), human (70 %), cattle (68 %), horse (68 %), and dog (61 %). This gene is structured in four exons and three introns as revealed by computer-assisted analysis. PCR-RFLP was established to detect the GU373669:c.854A > T substitution of porcine OSAP gene mRNA and association of this mutation with litter size traits was assessed in Large White (n = 100) and Landrace (n = 100) pig populations. Results demonstrated that this polymorphic locus was significantly associated with the litter size of all parities in Large White sows and Landrace sows (P < 0.05). Therefore, OSAP gene could be a useful candidate gene in selection for increasing litter size in pigs. These data serve as a foundation for further insight into this novel porcine gene.