The identification and use of probiotic bacterial stains is a practical approach to protect clams grown in aquaculture farms from disease. The inhibition of the pathogenic bacterium Vibrio alginolyticus was used as a trait to select a candidate probiotic bacterial strain in this study. An ideal bacterial strain, SW-1, was isolated from seawater from a clam farm. The selected isolate SW-1 was identified based on its physiological, morphological, and biochemical characteristics and its 16S rDNA sequence. The experiments showed that strain SW-1 had a high similarity to Pseudoalteromonas piscicida and could inhibit the growth of V. alginolyticus (V.-MP-1). SW-1 also improved the survival of clams following challenge with the pathogenic V.-MP-1. The mortality of clams was 100% after infection with 108 CFU/mL of V. alginolyticus, whereas mortality was only 11% when clams were infected with 108 CFU/mL of V.-MP1 while simultaneously exposed to the same concentration of Pseudoalteromonas SW-1, indicating that Pseudoalteromonas SW-1 could be used as a probiotic to protect farmed clams, and thus reduce the effects of antibiotics on aquatic environment.