The synthesis of twenty seven novel pyrazole derivatives bearing aryl substituted groups at positions 1 and 3 of the pyrazole structural motif and various functional groups at position 4 is presented. The critical step for their synthesis is the TCT/DMF promoted cyclization of the corresponding hydrazine precursors, which provided the desired pyrazole skeleton. The anticancer properties of the novel pyrazole derivatives were evaluated in vitro against human prostate (DU145), melanoma (A2058) and breast cancer (MCF-7) cell lines. Among the compounds tested, pyrazole 5a and its methoxy derivatives 3d,e were assayed as the most potent, displaying selective activity against the MCF-7 cell line with IC50 values of 14, 10 and 12 μM respectively. Results herein indicate that the reported backbone represents a promising structural lead for further development as antitumor agents.