Human tumors of various tissue origins show an intriguing overexpression of genes not considered oncogenes, such as that encoding Troponin-I (TnI), a well-known muscle protein. Out of the three TnI genes known in humans, the slow form, TNNI1, is affected the most. Drosophila has only one gene that encodes all forms of TnI, wupA. Here, we carried out a study on excess- and loss-of function conditions for wupA in Drosophila, and assayed TNNI1 down regulation in human tumors growing in mice. Drosophila TnI excess-of-function causes tissue overgrowth and potentiates the phenotypes due to oncogenic mutations in Ras, Notch and Lgl genes. By contrast, TnI loss-of-function reduces proliferation and antagonizes the overgrowth due to these oncogenic mutations. The conserved Hippo signaling pathway requires the up-regulation of TnI for tumor overgrowth. Troponin-I defective cells exhibit aberrant number of centrosomes, multiple double strand DNA breaks and undergo Flower-dependent cell competition in the wing disc. Human tumor cell lines treated with a human Troponin-I peptide arrest in G0. In addition, the proliferation of non-small-cell lung carcinoma xenografts in mice is restrained by down-regulation of the TNNI1 gene. Thus, Troponin-I reveals as a novel oncogene that links abnormal cell signaling, proliferation, genome integrity and tumor growth, and may open the possibility for a novel therapeutic target/marker.