Effect of thermo-photoperiod conditions on pre-flowering phasic development, number of unfolded leaves, verticillaster dry weight at flowering (VDW_FL) and grain yield were evaluated for chia (Salvia hispanica L.). The objectives of this study were to: (i) characterise the pre-flowering response of chia to thermo-photoperiod conditions during and pre- and post-inductive subphases; and (ii) determine the relationships between the duration of pre-flowering subphases, the VDW_FL and grain yield. Sowing-date experiments were conducted during three consecutive growing seasons with two chia genotypes under non-stressed conditions. Responses to thermo-photoperiod conditions were characterised by fitting a bi-linear model. All pre-flowering subphases showed a quantitative short-day response with a decrease in sensitivity to photoperiod in the later post-inductive subphase. The duration of the pre-inductive subphase was associated with the number of accumulated unfolded leaves, whereas the durations of the post-inductive subphases were not determined by the number of accumulated unfolded leaves. Higher VDW_FL was achieved when durations of the pre-flowering subphases increased. In addition, increases in the VDW_FL explained most of the variation in grain yield along the thermo-photoperiod conditions. Accordingly, this study suggests that developmental responses of chia and their relationships with VDW_FL and grain yield should be taken into account for grain-yield improvement of this orphan crop.