The swimming crab (Portunus trituberculatus) is an economically important aquaculture species in China. Mining candidate single nucleotide polymorphisms (SNPs) and genes associated with growth traits of P. trituberculatus is of great importance for the genetic breeding of this species. Among the genes associated with growth traits, “protein still life, isoforms C/SIF type 2 (SIF)” is thought to be an important gene affecting animal growth and development. In the current study, we investigated the SIF gene from P. trituberculatus to better understand its biological function. The full-length cDNA of SIF was 6185 bp, with a 5208 bp open reading frame encoding 1735 amino acids. Homology analysis showed that SIF shared 90.07% identity with Procambarus clarkii and 40.25–86.95% with other species. Expression pattern analysis of SIF in fast- and slow-growing groups revealed a significantly higher expression level in the former than in the latter (P