The novel YbNbO₄:Er³⁺ phosphors were firstly synthesized through the hydrothermal method by adding LiOH·H₂O as flux in the H₂O/EG system. YbNbO₄:Er³⁺ phosphors showed the agglomerated irregular polygons coexisting with some tiny grains. XRD and Raman spectra were measured to understand the phase structure and the crystal growth mechanism of YbNbO₄:Er³⁺ phosphors. The upconversion (UC) emission spectra, the pump power dependency and UC mechanism were studied under 980 nm excitation. Based on the fluorescence intensity ratio technique, YbNbO₄:Er³⁺ exhibited the maximum sensor sensitivity of 0.00712 K⁻¹ at 220 K, providing a promising application in optical low-temperature sensors.