The Bergman cyclization is one of the most fascinating rearrangements in chemistry, with important implications in organic synthesis and pharmacology. Here we demonstrate a reversible Bergman cyclization for the first time. We induced the on-surface transformation of an individual aromatic diradical into a highly strained ten-membered diyne using atomic manipulation and verified the products by non-contact atomic force microscopy with atomic resolution. The diyne and diradical were stabilized by using an ultrathin NaCl film as the substrate, and the diyne could be transformed back into the diradical. Importantly, the diradical and the diyne exhibit different reactivity, electronic, magnetic and optical properties associated with the changes in the bond topology, and spin multiplicity. With this reversible, triggered Bergman cyclization we demonstrated switching on demand between the two reactive intermediates by means of selective C-C bond formation or cleavage, which opens up the field of radical chemistry for on-surface reactions by atomic manipulation.