The effects of nitrogen (N) fertiliser rate and irrigation on seed yield and its components were evaluated for signal grass (Urochloa decumbens (Stapf) R.D. Webster; syn. of Brachiaria decumbens Stapf) cv. Basilisk in a field experiment in Umuarama, Paraná, Brazil. Two water regimes (irrigated and non-irrigated) and four nitrogen (N) fertiliser rates (0, 25, 50 and 75 kg ha^–1) were applied to perennial signal grass crops in a split-plot randomised complete block design with three replications. In two consecutive harvests, favourable rainfall resulted in irrigation having limited influence on most measurements, and the combined application of irrigation and N fertiliser did not improve seed yield. Compared with the nil N, the highest N application rate significantly increased seed yield for the first crop (266 vs 498 kg ha^–1) and the second crop (104 vs 286 kg ha^–1). Nitrogen fertilisation significantly increased number of seed per area, reproductive tiller density and plant biomass at harvest for the first and second crops. Harvest index, 1000-seed weight, reproductive tiller weight, number of spikelets per panicle and number of seeds per panicle were unaffected by N rate. Harvest index ranged from 1.10% to 3.63% and 1000-seed weight from 2.15 to 3.36 g. There were no treatment effects on number of days to flowering or anthesis. Fertilisation with 75 kg N ha^–1 for the first crop and 50–75 kg N ha^–1 for the second crop maximised signal grass seed yield.