Using an ytterbium-doped fiber with a 50 μm core and 0.028 NA, a pulse energy of 4.8 mJ was achieved directly from a single-mode Q-switched fiber laser. The repetition rate was 10 kHz and the average power was 48.4 W. The slope efficiencies with regard to the absorbed and launched pump power were ∼74% and ∼59% respectively. The pulse width decreased with increasing pump power. The 4.8 mJ pulse had a FWHM width of ∼300 ns. A shorter pulse of ∼200 ns FWHM was also achieved at 2.06 mJ in another configuration. The M2 was below 1.3 at all pulse energies. This work demonstrates record pulse energy directly from a single-mode Q-switched fiber laser and the feasibility of operating such a laser with high efficiencies.