Updated publication reference for PubMed record(s): 31286648. In eukaryotes, membrane contact sites (MCS) allow direct communication between organelles. Plants have evolved unique MCS, the plasmodesmata intercellular pores, which combine organelle tethering with regulation of cell-to-cell signalling, the molecular mechanisms of which remains unknown. Here, we identify Multiple C2 domains and Transmembrane region Proteins (MCTPs) as tethers that link the endoplasmic reticulum (ER) to the plasma membrane (PM) within plasmodesmata. We report that MCTPs, including MCTP3 and 4 recently identified as modulators of SHOOTMERISTEMLESS trafficking, are ER-anchored proteins that cluster at plasmodesmata. MCTPs insert into the ER via their transmembrane region whilst their C2 domains dock to the PM through interaction with anionic phospholipids. A mctp3/4 loss-of function mutant induces plant developmental defects while MCTP4 expression in a yeast .tether mutant partially restores ER-PM tethering. Our data suggest that MCTPs are unique membrane tethers controlling both ER-PM contacts and cell-cell signalling.