ABSTRACT Gibberellins (GAs) and auxin play central regulatory roles in seed germination and root system development, respectively, so that the application of these phytohormones to crops would be worthwhile, with an increasing potential demand in agriculture. However, there are few effective chemicals that simultaneously enhance both GA and auxin signaling. Here, we report on an artificial thiourea derivative chemical, Y21, that serves as both a GA‐signaling agonist and an auxin analog, promoting seed germination and root development, as well as low‐phosphorus tolerance. Phenotypic, biochemical, and genetic evidence demonstrated that Y21 enhances the interaction between GA and its receptor GID1C via the Val239 amino acid residue and consequently promotes degradation of the DELLA proteins REPRESSOR OF ga1‐3 (RGA) and RGA‐LIKE 2. Furthermore, we found that Y21 interacts with the auxin receptor TIR1 via the Cys405 residue and thus promotes the turnover of the auxin‐responsive Aux/IAA proteins. Consequently, Y21 significantly increases low‐phosphorus tolerance of treated plants by positively regulating lateral root development. To our knowledge, Y21 is the first GA‐signaling agonist to be identified, and our results also demonstrate that this potent synthetic chemical, identified by chemical genetic screening, is effective at modulating plant development and stress tolerance.