To provide a polytrimethylene terephthalate-based polyester that can produce a polytrimethylene terephthalate-based polyester fiber having improved light resistance.

The polytrimethylene terephthalate-based polyester is produced such that the polyester contains a manganese compound in an amount of 10-150 ppm in terms of a manganese element and that the amount of the manganese element and the amount of the phosphorus element contained in the polyester satisfy the relationship of numerical formula (1): 0≤P/Mn≤1 (wherein P is an amount of the phosphorus element expressed as a mol equivalent and Mn is an amount of the manganese element expressed as a mol equivalent).