Abstract A measurement of the atmospheric $${\nu }_{\mu }+{\bar{\nu }}_{\mu }$$<math><mrow><msub><mi>\nu </mi><mi>\mu </mi></msub><mo>+</mo><msub><mover><mrow><mi>\nu </mi></mrow><mrow><mo>¯</mo></mrow></mover><mi>\mu </mi></msub></mrow></math>fluxwithenergiesbetween1and100GeVispresented.ThemeasurementhasbeenperformedusingdatatakenwiththefirstsixdetectionunitsoftheKM3NeT/ORCAdetector,referredtoasORCA6.ThedatawerecollectedbetweenJanuary2020andNovember2021andcorrespondto510daysoflivetime,withatotalexposureof433kton$$\cdot$$<math><mo>\cdot </mo></math>years.Usingmachinelearningclassification,3894neutrinocandidateeventshavebeenselectedwithanatmosphericmuoncontaminationoflessthan1$$\%$$<math><mo>$${\nu }_{\mu }+{\bar{\nu }}_{\mu }$$<math><mrow><msub><mi>\nu </mi><mi>\mu </mi></msub><mo>+</mo><msub><mover><mrow><mi>\nu </mi></mrow><mrow><mo>¯</mo></mrow></mover><mi>\mu </mi></msub></mrow></math>energyspectrumisderivedusinganunfoldingprocedureandtheimpactofsystematicuncertaintiesisestimated.Theatmospheric$${\nu }_{\mu }+{\bar{\nu }}_{\mu }$$ ${\nu }_{\mu }+{\overline{\nu }}_{\mu }$ flux measured using the ORCA6 configuration is in agreement with the values measured by other experiments.