DNA is a remarkable polymer that can be manipulated by a large number of molecular tools including enzymes(1). A variety of geometric objects, periodic arrays and nanoscale devices have been constructed(2-13). Previously we synthesized dendrimer-like DNA and DNA nanobarcodes from branched DNA via ligases(14,15). Here we report the construction of a hydrogel entirely from branched DNA that are three-dimensional and can be crosslinked in nature. These DNA hydrogels were biocompatible, biodegradable, inexpensive to fabricate and easily moulded into desired shapes and sizes. The distinct difference of the DNA hydrogel to other bio-inspired hydrogels ( including peptide-based, alginate- based and DNA (linear)-polyacrylamide hydrogels(16-20)) is that the crosslinking is realized via efficient, ligase-mediated reactions. The advantage is that the gelling processes are achieved under physiological conditions and the encapsulations are accomplished in situ-drugs including proteins and even live mammalian cells can be encapsulated in the liquid phase eliminating the drug-loading step and also avoiding denaturing conditions. Fine tuning of these hydrogels is easily accomplished by adjusting the initial concentrations and types of branched DNA monomers, thus allowing the hydrogels to be tailored for specific applications such as controlled drug delivery, tissue engineering, 3D cell culture, cell transplant therapy and other biomedical applications.