Tool for mounting a tire onto a rim, suitable for making it easier to insert the bead of the tire under the bead-stopping edge of the rim and inside the channel of the rim itself; said tools comprises a body equipped with a fixed jaw and with a mobile jaw to which actuation means are operatively connected suitable for taking said mobile jaw closer to said fixed jaw to lock said body to the edge of the rim, a base intended to be rested on the sidewall of the tire being associated with said body, said base being fixedly connected to said body so as to be able to oscillate towards the axis of the rim to make it easier to insert the bead inside the bead-stopping edge of the rim.