In this study, a pyrazole-based (Py-KS) chemosensor was successfully designed and synthesized via Knoevenagel condensation and utilized as a colorimetric chemosensor for selective determination of Hg2+ in semi-aqueous media. The probe exhibits a typical absorption band at 519 nm upon interaction with Hg2+, which is responsible for remarkable color change from yellow to pink. The detection limit of Py-KS for the analysis of Hg2+ was found to be 4.73×10−7 M. The binding modes of interaction of Py-KS with Hg2+ have been determined by Job's plot which exhibits 2:1 (Py-KS: Hg2+) ratio, and further confirmed with 1H NMR titration and FT-IR analysis. The association constant of Py-KS-Hg2+ complex was found as 1.2×103 M−1 using Benesi-Hildebrand method. The probe could be recyclable after appropriate treatment with the reagents such as EDTA. Moreover, the designed probe Py-KS has successfully demonstrated for the detection of Hg2+ in real sample analysis. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim