AbstractLymphatic vascular development is regulated by well-characterised signalling and transcriptional pathways. These pathways regulate lymphatic endothelial cell (LEC) migration, motility, polarity and and morphogenesis. Canonical and non-canonical WNT signalling pathways are known to control LEC polarity and development of lymphatic vessels and valves.PKD1, encoding Polycystin-1, is the most commonly mutated gene in polycystic kidney disease but has also been shown to be essential in lymphatic vascular morphogenesis. The mechanism by whichPkd1acts during lymphangiogenesis remains unclear. Here we find that loss of non-canonical WNT signalling componentsWnt5aandRykphenocopy lymphatic defects seen inPkd1knockout mice. To investigate genetic interaction, we generatedPkd1/Wnt5adouble knockout mice. Loss ofWnt5asuppressed phenotypes seen in the lymphatic vasculature ofPkd1−/−mice and Pkd1 deletion suppressed phenotypes observed inWnt5a−/−mice. Thus, we report mutually suppressive roles forPkd1andWnt5a,with developing lymphatic networks restored to a more wild-type state in double mutant mice. This genetic interaction betweenPkd1and the non-canonical WNT signalling pathway ultimately controls LEC polarity and the morphogenesis of developing vessel networks. Our work suggests thatPkd1acts at least in part by regulating non-canonical WNT signalling during the formation of lymphatic vascular networks.