Keloid and hypertrophic scars are fibroproliferative disorders resulting from abnormal wound healing in genetically susceptible individuals. Current therapies are often ineffective. Kynurenine shows promise as a topical treatment for keloids and hypertrophic scars. In this study, healthy adult male and female subjects seeking treatment for mature keloid scars were enrolled. Subjects were randomized in double-blind fashion to receive kynurenic acid 0.5% (FS2) cream (Group 1), an active onion extract comparator treatment (Group 2), or the inactive vehicle (Group 3). Each treatment was applied twice-daily. Qualitative assessments were made using the Vancouver Scar Scale (VSS), as well as the Patient and Observer Scar Assessment Scales (POSAS). Among subjects in Group 1, there was a substantial decrease in mean PGSS scores after 30 days of treatment that continued to trend downward, becoming significant versus Group 2 at days 90 and 180 (P<0.05) and versus Group 3 at day 180 (P<0.01). Based on mean VSS scores, subjects in Group 1 achieved beneficial effects that became significant versus Group 2 at day 90 (P<0.01), day 120 (P<0.05), and day 180 (P<0.001) and versus Group 3 at day 180 (P<0.05). There were no significant improvements in Groups 2 or 3. There were no adverse events or local skin reactions. The twice-daily application of FS2 Cream represents a potentially new and effective treatment for mature keloid scars. J Drugs Dermatol. 2021;20(9):964-968. doi:10.36849/JDD.6197.