Partial nitritation for a low-strength wastewater at low temperature was stably achieved in an aerobic granular reactor. A bench-scale granular sludge bioreactor was operated in continuous mode treating an influent of 70 mg N-NH₄⁺ L^-1 to mimic pretreated municipal nitrogenous wastewater and the temperature was progressively decreased from 30 to 12.5 ºC. A suitable effluent nitrite to ammonium concentrations ratio to a subsequent anammox reactor was maintained stable during 300 days at 12.5 ºC. The average applied nitrogen loading rate at 12.5 ºC was 0.7±0.3 g N L^-1 d^-1, with an effluent nitrate concentrationof only 2.5±0.7 mg N-NO₃^- L^-1. The biomass fraction of nitrite-oxidizing bacteria (NOB) in the granular sludge decreased from 19% to only 1% in 6 months of reactor operation at 12.5 ºC. Nitrobacter spp. where found as the dominant NOB population, whereas Nitrospira spp. were not detected. Simulations indicated that: (i) NOB would only be effectively repressed when their oxygen half-saturation coefficient was higher than that of ammonia-oxidizing bacteria; and (ii) a lower specific growth rate of NOB was maintained at any point in the biofilm (even at 12.5 ºC) due to the bulk ammonium concentration imposed through the control strategy.