Mn4+-Doped　fluoride　red　phosphors　with　excellent　moisture-resistant　properties　and　high　emission　efficiency　are　highly　desired　for　warm　white　LEDs　and　wide　gamut　liquid　crystal　displays.　Herein,　we　synthesized　a　K2NaGaF6:Mn4+　red　phosphor　through　a　co-precipitation　method　at　room　temperature.　The　crystallographic　structure　of　the　K2NaGaF6　host　was　determined　based　on　single　crystal　X-ray　diffraction　data　for　the　first　time.　A　high　internal　photoluminescence　quantum　yield　of　82%　was　achieved　for　the　1.81　at%　K2NaGaF6:Mn4+　sample.　Through　low-temperature　and　high-resolution　spectroscopic　analysis,　we　revealed　that　Mn4+　ions　occupy　at　least　four　distinct　sites　in　the　K2NaGaF6　crystal.　One　of　them　is　assigned　to　the　Ga3+　site　with　high　O-h　site　symmetry　and　the　other　three　ones　possess　low　non-centrosymmetric　site　symmetries.　By　employing　5.51　at%　K2NaGaF6:Mn4+　as　a　red　phosphor,　a　warm　white　LED　with　a　low　correlated　color　temperature　of　3760　K,　a　high　color　rendering　index　of　92　(R-a)　and　a　luminous　efficacy　of　156　lm　W-1　was　packaged.　Aging　tests　at　85　degrees　C　and　85%　relative　humidity　on　the　fabricated　LEDs　demonstrated　that　K2NaGaF6:Mn4+　has　a　better　moisture-resistant　property　than　that　of　the　popular　K2SiF6:Mn4+　phosphor.　These　findings　demonstrate　the　great　potential　of　K2NaGaF6:Mn4+　as　a　highly　efficient　red　phosphor　with　excellent　moisture-resistant　performance.