The NH4Mn(AsO4). H2O compound was prepared by hydrothermal synthesis and characterized by spectroscopic and magnetic techniques. Its crystal structure was determined by powder X-ray diffraction data, using direct methods, and refined by the Rietveld method. The compound crystallizes in the orthorhombic system with space group Pmn2(1) and unit cell parameters a 5.840(1), b = 9.063(1), and c = 4.987(1) Angstrom. The EPR spectra of polycrystalline samples are isotropic with a g-value of approximately 2.0. The intensity of the signal increases up to 30 K, then decreases and is not detectable below 10 K. Magnetic measurements show antiferromagnetic interactions, with an estimated intralayer exchange parameter J = -0.9 K. Nonetheless, a weak ferromagnetism intrinsic to the sample was observed below T-N temperature.