Stiff, strong and tough ultrafine polyethylene fibers that rival the best high-performance fibers, but with diameters less than one micron, are fabricated for the first time by "gel-electrospinning." In this process, solution concentration and process temperatures are chosen to induce the formation of gel filaments "in flight," which are subsequently drawn at high rates as a consequence of the whipping instability. The resulting submicron-diameter fibers exhibited Young's moduli of 73 ± 13 GPa, yield strengths of 3.5 ± 0.6 GPa, and toughnesses of 1.8 ± 0.3 GPa, on average. Among the smallest fibers examined, one with a diameter of 490 ± 50 nm showed a Young's modulus of 110 ± 16 GPa, ultimate tensile strength of 6.3 ± 0.9 GPa, and toughness of 2.1 ± 0.3 GPa, a combination of mechanical properties that is unparalleled among polymer fibers to date. The correlation of stiffness, strength and toughness with fiber diameter is attributed to high crystallinity and crystallite orientation, combined with fewer defects and enhanced chain slip associated with small diameter and high specific surface area. Gel-electrospinning improves the prospects for production of such fibers at scale.