We measured Ni partitioning between olivine and melt, D^(ol/liq)_(Ni), in experiments on mid-ocean ridge basalt (MORB) encapsulated in olivine at pressures from 1 atm to 3·0 GPa and temperatures from 1400 to 1550°C. We present a series of experiments where the temperature (T) at each pressure (P) was selected so that the liquid composition remained approximately constant over the entire P–T range. This approach allowed us to investigate the effects of T and P on D^(ol/liq)_(Ni), independent of substantial changes in liquid composition. Our experiments show that for a liquid with ∼18 wt% MgO, D^(ol/liq)_(Ni) decreases from 5·0 to 3·8 as the temperature increases from 1400 to 1550°C. Fitting our experimental results and literature data to thermodynamic expressions for D^(ol/liq)_(Ni) as a function of both temperature and liquid composition shows that the small variations in liquid composition in our experiments account for little of the observed variation of D^(ol/liq)_(Ni). Because the changes in volume and heat capacity of the exchange reaction MgSi_(0-5)O^(ol)_2 + D^(ol/liq)_(Ni) ↔ NiSi_(0-5)O^(ol)_2 + MgO^(liq) are small, D^(molar)_(Ni), the Ni partition coefficient on a molar basis, is well described by In(D^(molar)_(Ni))=-^(Δ_rHo_(T_(ref)),P_(ref)/_(RT) + ^Δ_rSo_(T_(ref),P_(ref))/_R - In (X^(liq)_(MgO)/X^(ol)_(MgSi)_(0-5)O_2) with Δ_rH^o_T_(ref),_P_(ref)/_R = 4375 K and Δ_rSo_T_(ref),_P_(ref)/_R = –2·023 for our data (Δ_rH^o_T_(ref),_P_(ref)/_R = 4338 K and Δ_rSo_T_(ref),_P_(ref)/_R = –1·956 for our experiments combined with a compilation of literature data). This expression is easy to use and applicable to a wide range of pressures, temperatures, and phase compositions. Based on our results and data from the literature, the temperature dependence of D^(ol/liq)_(Ni) leads to the prediction that when a deep partial melt from a peridotitic mantle source is brought to low pressure and cooled, the first Mg-rich olivines to crystallize can have significantly higher NiO contents than those in the residual source from which the melt was extracted. This enrichment in Ni is driven by the difference between the temperature of low-pressure crystallization and the temperature of melt extraction from the residue. The average observed enrichment of Ni in forsteritic olivine phenocrysts from Hawaii—relative to the typical olivines from mantle peridotites—is consistent with a simple scenario of high-temperature partial melting of an olivine-bearing source at the base of the lithosphere followed by low-temperature crystallization of olivine. The most extreme enrichments of Ni in Hawaiian olivine phenocrysts and the lower Ni contents of some olivines can also be explained by the known variability of Ni contents of olivines from mantle peridotites via the same simple scenario. Although we cannot rule out alternative hypotheses for producing the high-Ni olivines observed in Hawaii and elsewhere, these processes or materials are unnecessary to account for NiO enrichments in olivine. The absolute temperature, in addition to the difference between the temperature of melt segregation from the residue and the temperature of low-pressure crystallization, is a significant factor in determining the degree of Ni enrichment in olivine phenocrysts relative to the olivines in the mantle source. The moderate Ni enrichment observed in most komatiitic olivines compared with those of Hawaii may result from the higher absolute temperatures required to generate MgO-rich komatiitic melts. Observed NiO enrichments in early crystallizing komatiitic olivine are consistent with their high temperatures of crystallization and with a deep origin for the komatiite parental melts.