A novel marine Alteromonas gracilis siphovirus, phage PB15, was isolated from the surface water of the Yellow Sea in August 2015. It has a head diameter of 58 ± 5 nm head and a contractile tail approximately 105 ± 10 nm in length, and overall the morphology suggests that PB15 belongs to the family Siphoviridae. PB15 phage is stable at over the temperature range 0-60oC. The best MOI of these phage was 0.1 and infectivity decreased above 60oC. The results suggest that phage is stable at pH value ranging between 3.0 and 11.0. Chloroform test shows that PB15 is not a lipid-containing phage. A one-step growth curve with a strain of A. gracilis gave a latent period of 16 minutes and rise period of 24 minutes and burst size of 60 PFU/cell. Genomic analysis of PB15 reveals a genome size of 37,333bp with 45.52% G+C content, and 61 ORFs. ORF sequences accounted for 30.36% of the genome sequence. There is no obvious similarity between PB15 and other known phages by genomic comparison using the BLASTN tool in the NCBI database.