Under 980 nm excitation, the temperature dependence of fivephoton UV (256 and 276 nm) upconversion luminescence in Yb³⁺-Er³⁺ codoped β4-NaLuF4 nanocrystals was studied from 303 K to 523 K. The ⁴D7/2 and ⁴G9/2 levels of Er³⁺ are confirmed to be thermally coupled levels. They are the highest energy states for optical thermometry known so far. By using fluorescence intensity ratio technique, optical temperature sensing characteristics based on the ⁴D7/2/4G9/24I15/2 transitions of Er³⁺ were reported here for the first time. The obtained sensitivity of this UV-based sensor is higher than that of green-based optical thermometer in low temperature range.