The present study aimed to evaluate the breeding biology of Rusty-collared Seedeater (Sporophila collaris) from the grasslands in southern Brazil. Data from 52 nests were collected during the 2015–2016 breeding season. Breeding started in mid-September with a peak in early December and extended to early April, a period of ∼203 d, which is clearly longer than the period recorded for Argentine populations. Only females built the nests, which were open cups. The nests were built in herbaceous and shrubby plants at an average height of 116.3 ± 40.6 cm; 85% were located above the ground and 15% over water. The most common clutch size was 2 eggs (n = 12 nests), followed by 3 eggs (n = 7 nests), with egglaying occurring on consecutive days. Only the female is in charge of incubation, spending an average of 12.8 ± 0.7 d on this activity. The average on-bout was 20.9 ± 15.92 min and the average off-bout was 12.23 ± 8.62 min. The hatching rate was 81% and the fertility rate was 90%. Both parents took care of the nestlings. Visiting frequency to the nest differed between the sexes (female = 5.67 visits/h, n = 170 visits; male = 1.83 visits/h, n = 55 visits; z = 2.80, P = 0.005). The nestlings remained in the nest for an average 11.1 ± 0.78 d. The apparent success was 25% and the nest success estimation varied from 13% (Mayfield method) to 29% (MARK).