We have built a laser-diode-pumped Nd:YAG laser that emits 5.5 W of power in a single-frequency, polarized, nearly diffraction-limited TEM(00) mode. The laser uses fiber-coupled pump lasers and a rectilinear geometry to achieve high efficiency in a simple side-pumped architecture. Injection locking produces a stable, single-frequency output with no intracavity frequency-selection elements.