High　optical　temperature　sensing　properties　based　on　rare-earth-doped　(K,Na)NbO3-based　ferro-/piezoelectrics　have　attracted　much　attention　due　to　their　potential　application　in　novel　optoelectronic　devices.　Here,　we　fabricated　Ho3+-doped　(K0.5Na0.5)NbO3–SrTiO3　transparent　ceramics　by　conventional　pressureless　sintering.　Their　microstructures,　transmittances,　up-conversion　photoluminescence,　and　optical　temperature　sensing　properties　have　been　characterized　in　details.　Because　of　the　cubic-like　phase,　dense,　and　fine-grained　structure　as　well　as　relaxor-like　feature,　the　ceramics　exhibit　high　transmittance　(~70%)　in　the　near-infrared　region.　Owing　to　Ho3+,　green　and　red　up-conversion　emissions　have　been　observed,　which　can　be　easily　modulated　by　temperature.　The　ceramics　have　stable　emission　colors　(<200°C)　and　superior　temperature-modulating　emission　color-tunable　performance　(>200°C).　Furthermore,　the　temperature　sensing　behavior　based　on　the　thermally　coupled　levels　(5F4,　5S2)　of　Ho3+　has　been　analyzed　by　a　fluorescence　intensity　ratio　technique.　The　transparent　ceramics　possess　outstanding　optical　temperature　sensitivity　(~0.0096/K　at　550 K),　higher　than　most　rare-earth-doped　materials　(e.g.,　ceramics,　glasses,　and　phosphors).　©　2019　The　American　Ceramic　Society