A simple hydrothermal method has been developed for the systematic synthesis of lanthanide orthophosphate crystals with different crystalline phases and morphologies. It has been shown that pure LnPO₄ compounds change structure with decreasing Ln ionic radius:  i.e., the orthophosphates from Ho to Lu as well as Y exist only in the tetragonal zircon (xenotime) structure, while the orthophosphates from La to Dy exist in the hexagonal structure under hydrothermal treatment. The obtained hexagonal structured lanthanide orthophosphate LnPO₄ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) products have a wirelike morphology. In contrast, tetragonal LnPO₄ (Ln = Ho, Er, Tm, Yb, Lu, Y) samples prepared under the same experimental conditions consist of nanoparticles. The obtained hexagonal LnPO₄ (Ln = La → Tb) can convert to the monoclinic monazite structured products, and their morphologies remained the same after calcination at 900 °C in air (Hexagonal DyPO₄ is an exceptional case, it transformed to tetragonal DyPO₄ by calcination), while the tetragonal structure for (Ho→ Lu, Y)PO₄ remains unchanged by calcination. The resulting LnPO₄ (Ln = La → Dy) products consist almost entirely of nanowires/nanorods with diameters of 5−120 nm and lengths ranging from several hundreds of nanometers to several micrometers. Europium doped LaPO₄ nanowires were also prepared, and their photoluminescent properties were reported. The optical absorption spectrum of CePO₄ nanowires was measured and showed some differences from that of bulk CePO₄ materials. The possible growth mechanism of lanthanide phosphate nanowires was explored in detail. X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, electron diffraction, infrared absorption spectra, X-ray photoelectron spectroscopy, optical absorption spectra, and photoluminescence spectra have been employed to characterize these materials.