The atypical cadherin FAT4 is known to regulate planar cell polarity and Hippo pathway signalling in a cell type-dependent manner. Recently, FAT4 mutations were identified in Hennekam syndrome, features of which include lymphedema and lymphangiectasia, uncovering a role for FAT4 in the lymphatic vasculature. Work from our group demonstrated two ways in which FAT4 regulates the lymphatic vascular development: 1) via establishing LEC polarity in response to flow and 2) via modulation of the VEGF-C/VEGFR3 signalling axis which is crucially important in lymphangiogenesis. Here, we show that FAT4 is also involved in the regulation of mitochondrial dynamics and metabolism in lymphatic endothelial cells (LEC). Using Airyscan microscopy and a custom macro tool for analysis of mitochondrial morphology, we found that human LEC treated with FAT4-targeting esiRNA display more fragmented mitochondria than control esiRNA treated cells. Furthermore, flow cytometry analysis of cells loaded with the fluorescent reactive oxygen species (ROS) sensor, CellROX Green, revealed increased levels of ROS in the absence of FAT4. Moreover, live imaging of LEC treated with an exogenous inducer of oxidative stress (tert-butyl hydroperoxide) showed that loss of FAT4 leads to faster release of ROS from the mitochondria to other cellular compartments. Mitochondrial dynamics were also altered in these conditions, as FAT4-deficient cells displayed less dynamic mitochondria with increased swelling over time. Interestingly, we also detected increased levels of epsin 1 in the absence of FAT4, confirming the previously described regulation of this protein by ROS (Wu et al., JCI 2018). Importantly, ROS-driven upregulation of epsins was shown to promote degradation of VEGFR3 and in turn, inhibit lymphangiogenesis in diabetic mice. These data establish a novel role for FAT4 in regulation of mitochondrial dynamics as well as regulating cellular levels and localisation of ROS, which likely influences key aspects of LEC biology including cell growth and metabolism.