Here, we established coumarin-30, a widely commercially available laser dye, as a new colchicine site binder that destabilizes microtubules in vitro and in cultured cancer cells at submicromolar concentrations. The combination of its potency as tubulin polymerization inhibitor and its fluorescent properties -such as high brightness, fluorescence enhancement upon tubulin binding and excitation/emission in the visible spectrum -makes this compound an excellent fluorescent probe for tubulin, supplementing the existing arsenal of fluorescent molecules that have already found numerous applications in fundamental biological research. For example, fluorescent analogs of eribulin and paclitaxel have recently been used to unravel novel microtubule lattice and tip properties. Allocolchicine, which binds to curved tubulin dimers, has been used to probe the conformation of tubulins in solution. Photoactivated analogs of paclitaxel and combretastatin have been utilized for light-controlled local stabilization or disassembly of the tubulin cytoskeleton in cells to study the role of microtubules in various cellular processes. Fluorescent peptides, likely targeting the C-terminal tails of tubulins, have been developed to visualize microtubule shafts, without perturbing microtubule dynamics. We envision that coumarin-30 may find diverse applications, including probing the conformations of free tubulins or the protofilaments at microtubule ends, and visualizing the interplay between the microtubule tip structure and microtubule dynamics.