Mechanisms of upconversion luminescence (UCL) of SrF2:Er phosphors corresponding to the ⁴G11/2 → ⁴I15/2, ²H9/2 → ⁴I15/2, ⁴F5/2 → ⁴I15/2, ⁴F7/2 → ⁴I15/2, ²H11/2 → ⁴I15/2, ⁴S3/2 → ⁴I15/2, ⁴F9/2 → ⁴I15/2, and ⁴I9/2 → ⁴I15/2 transitions upon excitation of the ⁴I11/2 level of Er³⁺ ions were investigated. Energy transfer upconversion processes are responsible for the populating of the ²H9/2, ²H11/2, ⁴S3/2, and ⁴F9/2 levels. Cooperative process is the dominant mechanism of luminescence from ⁴S3/2 and ⁴F9/2 levels for SrF2:Er with high concentrations of Er³⁺ ions. The UCL from ⁴G11/2 and ⁴F5/2 is explained by excited-state absorption. Cross-relaxation processes take part in the population of ⁴F9/2 and ⁴I9/2 levels. For quantifying material performance, the Er³⁺-concentration dependence of UCL and the absolute quantum yields of SrF2:Er were studied. The most intensive visible luminescence was obtained for SrF2:Er (14.2%) with 0.28% maximum quantum yield.