A　novel　light-induced　reversible　self-assembly　(LIRSA)　system　is　based　on　the　reversible　photodimerization　and　photocleavage　of　coumarin　groups　on　the　surface　of　gold　nanoparticles　(AuNPs)　in　THF　solution.　Facilitated　by　coumarin　groups,　light　irradiation　at　365　nm　triggers　the　stable　assembly　of　monodisperse　AuNPs;　the　resulting　self-assembly　system　can　be　disassembled　back　to　the　disassembled　state　by　a　relatively　short　exposure　to　benign　UV　light.　The　reversible　self-assembly　cycle　can　be　repeated　4　times.　A　specific　concentration　range　of　coumarin　ligand　and　the　THF　solvent　were　identified　to　be　the　two　predominant　factors　that　contribute　to　the　LIRSA　of　AuNPs.　This　is　the　first　successful　application　of　reversible　photodimerization　based　on　a　coumarin　derivative　in　the　field　of　AuNP　LIRSA.　This　LIRSA　system　may　provide　unique　opportunities　for　the　photoregulated　synthesis　of　many　adjustable　nanostructures　and　devices.　Colloidal　coumarin-functionalized　Au　nanoparticles　were　reversibly　self-assembled　based　on　coumarin　photolysis　in　response　to　light　irradiation.　Facilitated　by　coumarin　groups,　365　nm　light　irradiation　triggers　the　stable　assembly　of　monodisperse　Au　nanoparticles.　The　resulting　self-assembly　system　can　then　be　disassembled　through　a　relatively　short　exposure　to　benign　UV　light;　the　reversible　self-assembly　cycles　can　be　repeated　4　times.　©　2016　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim.