Nitrogen (N) deficiency has detrimental effects on productivity and the profit of producers in areas where signal grass [Urochloa decumbens (Stapf) R.D. Webster (syn. Brachiaria decumbens Stapf.)] cv. Basilisk is grown for seed production. The objective of this paper was to clarify the effects of indicators of signal grass plant N status on seed yield (SY), SY components, yield formation, seed quality, panicle growth parameters, and remobilization of vegetative N on seed growth. Germinable pure SY, harvest index (HI), and N harvest index (NHI) were also measured. Different rates of N fertilizer application (0, 50, 100, and 150 kg ha−1) were applied after the cleaning cut to both the first crop (October - January) and the second crop (February - May) in 2010–2011 and 2011–2012, on a sandy loam soil representative of soils used for seed production in Brazil. Although the N nutrition index (NNI) increased at key developmental stages, the highest values were near to 0.85. This suggests that all crops were maintained under N-limiting conditions. In N-limited crops, a strong relationship was detected between NNI and accumulated N deficit throughout the study period with relative SY. A low NNI after the cleaning cut was found to restrict fertile tiller number (FTN), spikelets per panicle, and spikelet density m−2 measured at anthesis. In all crops, at harvest, NNI at anthesis increased germinable pure SY, FTN, number of seeds per panicle, HI, NHI, and amount of remobilized N to seeds, but not thousand seed weight (TSW), seed germination, panicle dry matter (DM) accumulation rate, and individual seed growth rate. Regression analyses suggested that the NNI, accumulated N deficit, aboveground plant biomass (AGPB), and N content were better associated with relative SY than with plant N concentration (PNC). The study shows that the NNI quantifies the intensity and duration of N deficiency in signal grass and should be considered in research studies and for application in seed production fields to improve N fertilization recommendations.