The intrinsically disordered protein p15 PAF regulates DNA replication and repair by binding to the proliferating cell nuclear antigen (PCNA) sliding clamp. We present the structure of the human p15 PAF –PCNA complex. Crystallography and NMR show the central PCNA-interacting protein motif (PIP-box) of p15 PAF tightly bound to the front-face of PCNA. In contrast to other PCNA-interacting proteins, p15 PAF also contacts the inside of, and passes through, the PCNA ring. The disordered p15 PAF termini emerge at opposite faces of the ring, but remain protected from 20S proteasomal degradation. Both free and PCNA-bound p15 PAF binds DNA mainly through its histone-like N-terminal tail, while PCNA does not, and a model of the ternary complex with DNA inside the PCNA ring is consistent with electron micrographs. We propose that p15 PAF acts as a flexible drag that regulates PCNA sliding along the DNA and facilitates the switch from replicative to translesion synthesis polymerase binding.