Cells can sacrifice their individuality and fuse to acquire specialized functions. The generation of multinucleated syncytia in the developing skeletal myocytes by cell fusion is thought to be essential for effective contractility. However, the prevalence and significance of cell fusion in different tissues under developmental or physiological conditions is not well described. Here, we introduce a transgenic zebrafish reporter line that allows fluorescent labeling of polyploid cells. Surprisingly, besides skeletal muscles, some cardiomyocytes turn on the fluorescent reporter. This cell population largely contributes to the proliferating cardiomyocytes at this stage. Therefore, we hypothesized that, rather than cytokinesis failure, membrane fusion is the mechanism that activates the polyploidy reporter in these cells. Correspondingly, we observed a reduction of polyploidy reporter-activated cardiomyocytes in the cell fusion-compromised jam3b mutants, accompanied by a decrease in cardiomyocyte proliferation. Regenerative response of injured adult heart, conversely, increases polyploidy reporter-activated cells. Furthermore, blastula transplantations show a significant number of cardiomyocytes that harbor both donor- and host-derived transgenes, further suggesting the occurrence of cardiomyocyte fusion in the larval heart. Together, our findings not only uncover a novel cellular process occurring in the developing heart, but also highlight its possible role in cardiomyocyte proliferation in the vertebrate heart.