We report the synthesis and biological evaluation of a new series of oligosquaramide-based macrocycles as anticancer agents. Compound 7, considered as representative of this series, exhibited significant antiproliferative activity against the NCI-60 human tumor cell line panel, with IC(50) values ranging from 1 to 10 μM. The results show that sensitivity to cyclosquaramides is clearly dependent on cell type, underscoring a degree of biological selectivity. The observed antiproliferative effects appear to be related to deregulation of protein phosphorylation, as compounds 7 and 8 are effective inhibitors of several important kinases such as ABL1, CDK4, CHK1, PKC, c-MET, and FGFR, among others. The corresponding acyclic oligosquaramides and smaller cyclosquaramides did not show antitumor activity, suggesting that a macrocyclic structure with minimal molecular size plays a key role in the observed antitumor activity.