High-strength　steel　and　ultra-high-performance　concrete　(UHPC)　can　increase　structural　resistance　and　reduce　material　consumption　efficiently.　However,　current　design　specifications　do　not　permit　the　use　of　them　in　concrete-filled　steel　tube　members　due　to　the　lack　of　research.　To　address　it,　this　paper　experimentally　investigates　the　shear　behavior　of　square　ultra-high　performance　concrete-filled　high-strength　steel　tube　(referred　to　as　CuFTh　hereafter)　members.　A　total　of　20　CuFTh　specimens　were　tested,　considering　the　effects　of　the　span-todepth　ratio,　width-to-thickness　ratio　of　the　steel　tube,　yield　stress　of　steel,　and　fiber　volume　content　of　UHPC.　The　test　results　showed　that:　(i)　the　failure　mode　was　governed　by　the　shear　span-to-depth　ratio　(a/H),　including　shear　failure　(a/H　=　0.2　or　0.5)　and　combined　shear-flexural　failure　(a/H　=　0.8　or　1.0);　(ii)　the　shear　strength　increased　with　decreasing　shear　span-to-depth　ratio　and　the　width-to-thickness　ratio　of　the　steel　tube;　(iii)　increasing　the　yield　stress　of　steel　and　fiber　volume　content　of　UHPC　improved　the　shear　strength.　The　applicability　of　current　specifications　for　estimating　the　shear　strength　of　square　CuFTh　members　was　evaluated.　It　was　shown　that　CECS　28　(CECS　2012)　had　the　most　reasonable　estimation.