Transition　metal　chalcogenides　with　hollow　nanostructures　have　been　considered　as　promising　substitutes　as　precious　metal　electrocatalysts　for　energy　conversion　and　storage.　We　synthesized　NiCo2S4　double-shelled　ball-in-ball　hollow　spheres　(BHSs)　via　a　simple　solvothermal　route　and　applied　them　in　both　dye-sensitized　solar　cells　(DSSCs)　and　hydrogen　evolution　reactions　(HERs)　at　the　same　time,　which　were　clean　and　sustainable　ways　to　convert　energy.　Benefiting　from　their　remarkable　structure　features　and　advantageous　chemical　compositions,　NiCo2S4　BHSs　composed　of　tiny　crystals　possessed　large　surface　area,　well-defined　interior　voids,　and　high　catalytic　activity.　The　DSSC　with　NiCo2S4　BHSs　under　100　mW　cm-2　irradiation　possessed　a　power　conversion　efficiency　of　9.49%　(Pt,　8.30%).　Besides,　NiCo2S4　BHSs　as　a　HER　catalyst　also　possessed　a　small　onset　overpotential　(27.9　mV)　and　a　low　overpotential　(89.7　mV　at　10　mA　cm-2)　under　alkaline　conditions.　Therefore,　this　work　offers　a　sensible　strategy　to　synthesize　bifunctional　electrocatalysts　for　DSSCs　and　HERs.　©　2018　American　Chemical　Society.