AbstractExpansion into areas outside their centres of origin requires knowledge of quinoa's physiology and its response to the environment concerning the determination of seed yield and its numerical components to facilitate crop management and breeding. The objectives of the current work were to analyse the determination of seed weight (SW) of different quinoa genotypes adapted to temperate climates (Sea‐level type) under a wide range of growth conditions through: (a) the analysis of seed growth rate (SGR) and seed filling duration (SFD) variation, (b) the study of seed's water dynamics, and (c) the analysis of the effect of environmental conditions on SW components and water dynamics. For this, four genotypes were planted under four environments, as a result of the combination of sowing dates and growth conditions (field or greenhouse). Variation in SW was a result of a change in SGR (r2 = .66; p < .001) and not in SFD (r2 = .03; p = .45). Final SW was closely related to maximum seed water content (r2 = .84; p < .0001) achieved around the mid‐seed filling. In relation to climate variables, SWs showed a negative association with temperature in all genotypes. Of the SW components, SGR decreased with increases in temperature while SFD showed no association with it. Source–sink ratio during seed filling did not explain variation in SW, but the analysis of this relationship allowed us to identify the type of limitation experienced during seed filling in the different environments. From the perspective of improvement, breeding schemes should be focused on the increase of the source during seed filling (current or stored resources) in early sowings, and that in the sink (higher seed number) for late sowings.