The diffusion and trapping of deuterium in nickel irradiated by 3He was studied. One surface of a nickel membrane was exposed to deuterium plasma and the deuterium concentration near the surface was observed in situ using the D( 3He, p) 4He nuclear reaction. Although no trapped deuterium was observed initially, the amount increased toward a saturated value as the 3He irradiation continued. A diffusion model, in which only one kind of trap was assumed, was used to analyze the observed deuterium concentration. The saturated trap density beneath the surface was found to be 9.0 x10 −4 atom fraction at 523 K. The trap profile over the mean depth of the incident 3He ion was similar to the estimated distribution of atomic displacements. A trap density at 523 K of 0.0044 atom fraction corresponds to 1 displacement per atom.