Saline environments are a rich source for bacteria of potential applications. It was thus aimed in this study to evaluate the potentiality of ZnO NPs synthesized extracellularly by haloalkaliphilic  Alkalibacillus sp. w7 isolated from Al- Hamra salt lake at Wadi Al Natrun. This study is the first report on the application of Alkalibacillus sp. w7 ZnO NPs in dye photodegradation and as antimicrobial agents. The photocatalytic degradation of methylene blue and methyl red under simulated sunlight was 96.47% and 92.6%, respectively within 210 min. Antimicrobial activity of ZnO NPs was greater against tested Gram-negative bacteria and Candida albicans when compared to Gram-positive bacteria, with the highest activity against Escherichia coli (35 mm). The cellular morphological changes of E. coli under SEM showed that ZnO NPs can cause surface perturbation and form blebs and irregular pits on the cell wall. Furthermore, this study evinces photocatalytic and antimicrobial mechanisms of ZnO NPs by disruption of cellular functions and dyes via reactive oxygen species (ROS)-dependent generation of superoxide anion radical. Thus, the present work could promote a new strategy dealing with the pollution of synthetic dyes in aquatic ecosystems and the impact of Alkalibacillus sp. w7-mediated ZnO NPs as a novel antimicrobial agent.