Insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1) plays a crucial role in cellular growth, proliferation, and embryonic development by regulating mRNA stability. IGF2BP1 has been recently found to be located on a new 194.6-kb interval of the fecundity gene FecL. A total of nine insertion/deletion (InDel) variations within IGF2BP1 were identified in 916 adult Australian White sheep, and three were polymorphic: a 5-bp InDel in a downstream region, 9-bp InDel in intron 4, and 15-bp InDel in intron 2. Association analyses indicated that both the 5-bp and 9-bp InDel mutations were significantly associated with litter size at 2nd gestation. The heterozygous genotype (ID) of the 5-bp InDel locus and homozygous insertion genotype (II) of the 9-bp InDel locus were associated with large litter size. Moreover, the combined genotype ID-II of the 5-bp and 9-bp InDel genotypes was identified as the best genotype. These findings will pave the way for future research on marker-assisted selection of reproductive traits in sheep breeding.