Physical entanglement of polymer chains is an interaction that is believed to be too weak to build polymer networks for hydrogelation. Herein, we report a cluster strategy for preparing a class of fundamentally new hydrogels that are crosslinked by only entanglement interaction of polymer chains. The entanglement cluster is created by in situ polymerization of the monomer (acrylamide) in polyacrylamide nanogels to form long inter-connected polyacrylamide chains passing through nanogels. The as-prepared hydrogels can swell high content of water to achieve very low polymer fractions (∼1 wt%). Although they contain an ultrahigh water content and possess an ultra-soft nature (low to∼10 Pa storage modulus), these physical entanglement hydrogels (PEH) are tough and stretchable (>6 times) due to the free-sliding dynamics of the entanglement-cluster crosslinking interaction. Moreover, this free-sliding nature allows PEH to fully recover their mechanical properties after lyophilization–rehydration treatment. Because of their novel properties, PEH should be promising for various applications.