To　enhance　the　performance　of　semiconductor　photocatalysts,　cocatalysts　are　used　to　accelerate　surface　reactions.　Herein,　ultrasmall　molybdenum–oxygen　(MoO　x　)　clusters　are　developed　as　a　novel　non-noble　cocatalyst,　which　significantly　promotes　the　photocatalytic　hydrogen　generation　rate　of　CdS　nanowires　(NWs).　As　indicated　by　extended　X-ray　absorption　fine　structure　analysis,　direct　bonds　are　formed　between　CdS　NWs　and　MoO　x　clusters,　which　guarantee　the　migration　of　photo-generated　charge　carriers.　Moreover,　the　MoO　x　clusters　induce　deep　electron　trap　states　owing　to　the　unique　atomic　arrangement　and　configuration　with　the　generation　of　long-lived　electrons　to　enhance　the　activity.　These　findings　may　guide　the　design　of　efficient　cocatalytic　materials　for　solar　water　splitting　and　open　a　new　avenue　toward　practical　applications　of　ultrasmall　clusters.　©　2018　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim