Spirocyclic frameworks, polycyclic structures in which two rings share a single carbon atom, constitute key structural motifs across diverse areas, including optoelectronic and hole-transporting materials^1–3, ligand design for coordination complexes^4–6, and numerous natural products⁷. In a spiro skeleton, the two molecular units adopt an orthogonal arrangement, generating a rigid three-dimensional (3D) architecture. Introducing a single carbon atom to join two components to form the spiro centre therefore represents a direct strategy for constructing such frameworks. Previous carbon-atom-transfer approaches have largely relied on π-bond reactivity, whereas controlled insertion of a carbon atom into C–C σ-bonds remains challenging^8–11. Here we report a rhodium-catalysed spiro-carbon-atom insertion reaction that converts biphenylene substrates into spirocyclic structures through sequential C–C bond activation. Using N-isocyanoiminophosphorane (PINC)^12–14 as an atomic carbon equivalent, the reaction forms four new C–C bonds at a spiro centre. The transformation enables both homo- and cross-spirofusion of biphenylenes and can be extended to reactions combining σ- and π-bond activation. These results establish a catalytic strategy for constructing structurally complex spirocyclic frameworks through C–C σ-bond activation, with a single carbon atom serving as a central linchpin.