An improved window sill system has a modular bottom sub-sill unit to be attached to a bottom part of a window unit and mounted to a bottom frame part of a window frame in an opening in an exterior wall of a building in order to provide improved water drainage for the window sill system. The bottom sub-sill unit has a water trough or channel extending across its length at a front edge facing outwardly of a window unit to which the sub-sill unit is mounted. The water trough or channel has a top plate provided with a plurality of weep holes for capturing rainwater dripping down the window into the water trough or channel. A base plate is provided for mounting the bottom sub-sill unit with fastener screws to the bottom frame part of the window frame. An inner deflection wall deflects water from the weep holes into the water trough or channel to prevent water from reaching the fastener screws in a dry zone below the inner deflection wall.