A novel double network (DN) hydrogel with highly enhanced toughness has been prepared using reversible addition–fragmentation transfer (RAFT)-modified poly(2-acrylamide-2-methylpropane sulfonic acid) (PAMPS) as the first network, and polyacrylamide (PAM) as the second network. The mechanical properties of the first-network-modified PAMPS/PAM DN hydrogels have been studied and the new DN hydrogel shows remarkably high fracture energy (3.3 MJ m⁻³) in tensile deformation, which is nearly 9 times larger than that of the unmodified PAMPS/PAM DN hydrogel. Synchrotron radiation small-angle X-ray scattering (SAXS) was used to study the microstructures of the first-network single network (SN) and DN hydrogels. It was demonstrated by the SAXS results that the introduction of the RAFT agent into the first network enlarges the size of the ordered cross-linked domains in the SN hydrogel. The large ordered domains are beneficial for entanglement and interpenetration between the first and the second networks to dissipate concentrated stress more efficiently, resulting in the enhanced toughness of the first-network-modified DN hydrogels.