Electrically conductive yet mechanically strong composite hydrogels are highly desired in many applications, and graphene would be the most promising candidate for these hydrogels. While previous research utilized either as-prepared or reduced graphene oxide, we herein prepared high structural integrity yet solution-processable graphene sheets for the development of graphene/polymer composite hydrogels. A percolation threshold of electrical conductivity was found at 0.4 vol% graphene for these hydrogels. At 1.0 vol%, Young's modulus and compressive strength were respectively enhanced from 1.64 to 19.03 MPa and from 0.37 to 6.90 MPa, corresponding to 1060% and 1765% improvements. These improvements were explained from the perspectives of the graphene sheets' dispersion and their interaction with the polymer and water molecules. It is worth noting that the addition of graphene improved the hydrogel pH sensitivity. (C) 2016 Elsevier Ltd. All rights reserved.