The invention relates to a device for tensioning a canvas on a frame, which is for assembling two bars the ends of which comprise a groove. The device comprises: an optional sheath intended to be immobilized in a groove; a spacer comprising a central portion from which extend two lateral portions each having a bearing face coming to abut against the bottom of the sheath; a tensioner comprising a central portion from which extend securing means for securing the tensioner to the bars, via the sheath; a connecting member for connecting the spacer to the tensioner, and means for bringing the spacer closer to the tensioner, wherein the spacer being brought closer to the tensioner will cause each bar to translate along its longitudinal axis so as to space the bars apart from each other while keeping them perpendicular to each other.