The magnetic properties of Cu(NO3)2-2.5 H2O are dominated by the presence of antiferromagnetically coupled pairs of Cu ions with a singlet ground state separated by about 5.2 K from an excited triplet. Neutron-diffraction studies have been performed on a crystal of Cu(NO3)2-2.5 D2O between 0.08 and 0.25 K in fields of up to 60 kOe applied along the monoclinic b axis. At 0.125 K and between 28 and 44 kOe, weak magnetic scattering appears at some of the nuclear peak positions. The magnetic structure consists of alternating antiferromagnetic chains approximately in the a-c plane which are antiferromagnetically coupled to neighboring chains in the same plane. The antiferromagnetic axis is perpendicular to the field direction, lying roughly halfway between the a and c axes, and the moment attains a maximum of about 0.45 B per Cu atom at 38 kOe, falling off sharply close to the upper and lower critical fields. Magnetic scattering also appears at a different set of nuclear peak positions above 28 kOe, reaching a saturation value at about 46 kOe. This results from an induced ferromagnetic component along the b axis with a saturation moment of roughly 0.9 B per Cu atom. The results are in accordance with numerous previous studies, and in particular allow a choice between two antiferromagnetic structures proposed by Diederix et al. from proton resonance measurements.