The present invention provides a polytrimethylene terephthalate monofilament yarn excellent in softness, bending recovery or durability against a long term use. The inventive polytrimethylene terephthalate monofilament yarn is constituted by trimethylene terephthalate composed of 90 mol % or more of trymethylene terephthalate repeating units and 10 mol % or less of other ester repeating units, wherein the yarn has a single-fiber size of 50 dtex or more, an intrinsic viscosity in a range from 0.8 to 1.3 dl/g, and a boiling water shrinkage of 2% or less.