One of the ways to reduce chemical fertilizer application is the use of cover crops, which improve soil properties and supply nutrition to subsequent crops. The application effect of a legume cover crop, hairy vetch (Vicia villosa R., HV), on N dynamics in fresh market tomatoes (Solanum lycopersicum L.), ‘House Momotaro,’ was investigated using the ¹⁵N-labeling method. Tomato seedlings were transplanted into a 1/2000 a Wagner pot at 0, 80, and 240 kg·ha⁻¹ of N application (N0HV, N80HV, and N240HV) on June 9, 2011. Before transplanting, the labeled HV and chemical fertilizers were incorporated into the soil. Five tomato plants were collected 6 times in each treatment and then separated into leaves, stems, and roots. Fruits were harvested at maturity. HV-derived N uptake was recognized mainly in the first 4 weeks after transplant (WAT). Especially in N240HV, HV-derived N uptake ceased at 4 WAT. The uptake amounts of HV-derived N at 10 WAT were 587, 657, and 729 mg·plant⁻¹ in N240HV, N80HV, and N0HV, respectively, and were increased by decreasing N fertilizer application. The rate of N uptake derived from HV to total N uptake in tomato plants (%N_dfhv) was the highest at 2 WAT, and %N_dfhv in N80HV (52.1%) and N0HV (51.5%) were significantly higher than in N240HV (43.6%). After 2 WAT, %N_dfhv decreased gradually in all N rates as tomatoes grew and decreased to 24.8%, 34.4%, and 37.1% in N240HV, N80HV, and N0HV, respectively, until 12 WAT. Nitrogen use efficiency (NUE) from HV-derived N was the highest at 10 WAT, and N0HV (55.3%) was significantly higher than N240HV (44.5%) and N80HV (49.8%). The partition rate of HV-derived N into fruits was 63.9%, and 39.7% of HV-derived N was distributed into 1st and 2nd fruit clusters. From these results, it was clarified that HV can be expected to be an alternative N fertilizer because HV-derived N was absorbed effectively with a small amount of N fertilizer. Further research on fertilizer management in tomato’s early stage will be needed for an N-reduction system because HV-derived N was mainly absorbed for 4 WAT.