Abstract: A roofing membrane heat seal indicator provides a positive, visually perceptible indication that a joint or seam has been raised to a sufficiently high temperature to achieve a proper seal. The heat seal indicator is applied adjacent and parallel to an edge of a roofing membrane which will be arranged on a roof to form the upper (exposed) surface of an overlapping membrane seam. As the adjacent surfaces of the overlapped membranes are heated, heat passes through the membrane to the heat seal indicator. The heat seal indicator may be a temperature sensitive ink pigmented with material that changes color upon being subjected to a temperature indicating that the membrane surface has been subjected to a sufficiently high temperature to achieve an appropriate seal. Alternatively, the indicator may be an adhered layer of a ground or flaked polymer, such as polyethylene, which is initially opaque but melts to become translucent, thereby indicating obtainment of a proper sealing temperature.