The search for new growth-regulating compounds remains a relevant research direction in view of the issue of food security. Previously, a number of thiazolyl acetic acid derivatives have been synthesized, which are promising biologically active compounds according to their physicochemical characteristics. The aim of this work was to study the growth-regulating properties of both previously and newly synthesized thiazolyl acetic acid derivatives using a growth phytotest with Lepidium sativum and Sinapis alba. This study was carried out under laboratory conditions of phytotesting growth indicators of test plants of the class Dicotyledones—L. sativum and S. alba under the influence of 10 μg/mL, 100 μg/mL, and 1000 μg/mL aqueous solutions of the compounds with a comparative analysis of the action of the active components of Rhizopon^TM and herbicide benazolin. It was found that, exhibiting high herbicidal properties at a concentration of 1000 μg/mL (primarily the compound with a benzothiazole fragment), the studied thiazolyl acetic acid derivatives with decreasing concentration (100 μg/mL and 10 μg/mL) reduce phytotoxicity to its complete elimination against L. sativum and S. alba for the compound 2-[2-(N-tert-butoxycarbonyl)-aminoethyl-1,3thiazol-4-yl]acetic acid (compound 3), as well as against L. sativum for the compound 5-phenylthiazol-2-yl acetic acid (compound 5). A weak stimulating effect on the length of the above-ground part of S. alba seedlings was recorded for 2-[2-(3,3,3-trifluoropropyl)-1,3thiazol-4-yl]acetic acid (compound 1) and 2-[2-(N-tert-butoxycarbonyl)-aminoethyl-1,3thiazol-4-yl]acetic acid (compound 3) at their concentration of 10 μg/mL; however, an improvement in the vitality index under the action of these compounds was not observed. The results obtained in this study indicate the biological activity of both previously and newly synthesized thiazolyl acetic acid derivatives, namely their growth-regulatory properties, expanding knowledge about promising herbicidal compounds with a possible hormetic effect, which requires further research.