Urochloa (syn. Brachiaria) cultivars are widely used as forage for ruminants in tropical countries and also in Thailand. Two new Urochloa cultivars, OKI‐1 and Br‐203, have high forage yield and digestibility compared with other cultivated Urochloa grasses. Seed production is a fundamental requirement for widespread sowing, and it can be increased by optimizing agronomic practices, such as changing plant spacing. Two field trials were conducted in 2018–2020 at Lampang Animal Nutrition Research and Development Center, Northern Thailand, to determine the optimal plant spacing for seed production of these Urochloa cultivars. In both trials, four plant‐spacing regimes (50 × 100, 75 × 100, 100 × 100 and 125 × 100 cm) were used in a randomized complete block design with four replicates. In two consecutive harvests of both trials, plant spacing did not significantly affect seed yield and quality but affected tiller and inflorescence densities. Specifically, significant higher numbers of tillers and inflorescences were produced per unit area at the narrowest plant spacing (i.e., 50 × 100 cm). OKI‐1 and Br‐203 tended to have higher pure seed yields (PSY) at plant spacings of 100 × 100 (136.46 kg/ha) and 75 × 100 cm (79.59 kg/ha), respectively. Both cultivars showed similar trends in PSY, filled seed percentage and a thousand seed weight, which tended to be higher in the first‐year crops than the second‐year crops. This difference could be attributed to a reduction in available soil N resulting from a large amount of N utilization during vegetative growth, combined with inadequate and erratic rainfall. In addition to the recommended optimum plant spacing, sufficient fertilizer and suitable environmental factors could increase seed yield. [ABSTRACT FROM AUTHOR]