Low　polyhydroxyalkanoate　(PHA)　volumetric　productivity　from　wastewater　limits　low-cost　PHA　production.　To　resolve　this　problem,　an　external　magnetic　field　(MF)　coupled　with　upshock　salinity　was　applied　to　PHA　production　by　Haloferax　mediterranei　(family　Halobacteriaceae).　Elevating　the　fermentation　salinity　over　the　optimal　growth　salinity　(200　g/L)　increased　the　PHA　cell　content　while　inhibiting　cell　proliferation,　decreasing　volumetric　productivity.　When　a　MF　of　50　mT　in　300　g/L　salinity　was　applied,　H.　mediterranei　proliferation　and　PHA　cell　content　were　promoted,　leading　to　a　7.95%　increase　in　PHA　volumetric　productivity　in　synthetic　molasses　wastewater　and　a　13.82%　increase　in　glucose　feeding　compared　with　those　in　200　g/L　salinity.　Under　the　MF,　osmotic　pressure　regulation　was　activated　by　accumulating　K1　and　increasing　betaine　synthesis.　The　maximum　betaine　content　increased　by　74.33%　in　300　g/L　salinity　with　a　50-mT　MF　compared　with　that　in　200　g/L　salinity.　When　a　50-mT　MF　in　300　g/L　salinity　was　applied,　the　malondialdehyde　(MDA)　content　decreased　by　32.66%　and　the　activity　of　superoxide　dismutase　(SOD)　increased　by　46.89%,　which　reduced　the　oxidative　damage.　This　study　provides　a　new　solution　to　enhance　PHA　volumetric　productivity　by　MF　and　an　insight　into　the　magnetic　effects　of　H.　mediterranei.　IMPORTANCE　The　obstacle　to　replacing　petroplastics　with　PHA　is　its　high　production　cost.　To　increase　the　fermentation　economy,　a　novel　strategy　of　coupling　a　MF　with　salinity　upshock　was　applied,　which　enhanced　the　PHA　volumetric　productivity　of　H.　mediterranei　in　fermenting　molasses　wastewater.　The　magnetic　effect　of　H.　mediterranei　was　found　at　a　MF　of　50　mT,　which　improved　the　salt　tolerance　of　H.　mediterranei　and　reduced　the　oxidative　damage　induced　by　the　elevated　salinity,　thereby　promoting　proliferation　and　PHA　cell　content.　This　is　the　first　time　a　technical　method　for　enhancing　PHA　volumetric　productivity　by　means　of　a　MF　has　been　proposed.　Such　a　strategy　can　advance　the　utilization　of　H.　mediterranei　for　the　industrial　production　of　PHA　using　organic　wastewater.　Copyright　©　2022　American　Society　for　Microbiology.　All　Rights　Reserved.