The behavior of alkali-activated ultra-high-performance concrete (A-UHPC) at elevated temperatures is unknown. This study addresses this gap by investigating the behavior of A-UHPC under varying temperatures with steel fiber additions (1%, 2%, and 3%), and considering target temperatures (20 °C, 200 °C, 400 °C, 600 °C, and 800 °C) as design variables. As the results, A-UHPC with steel fibers showed improved fire resistance, suffering less compressive strength loss at 800 °C than fiber-free A-UHPC. High temperatures initially optimized A-UHPC’s microstructure at 200 °C but later caused damage through microstructure propagation. Steel fibers enhanced A-UHPC’s ductility, resulting in ductile failure even at 800 °C. A-UHPC exhibited a unique mechanical degradation pattern under elevated temperatures, distinct from ordinary cement-based concrete. Empirical models accurately predicted its behavior, offering valuable insights for engineers dealing with heavy loads and high temperatures.