Twenty-five 3-substituted isocoumarins were synthesized using cutting edge microwave-assisted technology in high yields. The syntheses of different isocoumarins were carried out in a single step by the direct condensation of homophthalic acid with aryol and acyl chlorides under the solvent-free conditions without any solid support. The structures of all the synthesized compounds were characterized using different spectroscopic techniques including UV, IR, 1HNMR and EIMS and purity was confirmed by CHN analysis. All the synthesized compounds were tested for in vitro leishmanicidal activity. Compounds 3a, 3b, 3g, 3l, 3m, 3r, 3t, 3w, 3x, and 3y displayed potential in vitro leishmanicidal activity with IC50 values in the range of 0.56-84.38 μg/ml, whereas standard inhibitors amphotericine B have IC50 = 0.24 μg/ml. The compounds 3b, 3g, 3m, 3t, 3w, 3x, and 3y having IC50 values 27.86, 28.88, 36.49, 34.37, 28.68, 0.89 and 0.56 μg/ml, respectively, were most active among the present series while remaining others were found less active. The compound 3x and 3y can act as potential lead molecules for further development of isocoumarin-based new drugs for the treatment of leishmaniasis.