A novel nanohybrid hydrogel was in situ prepared by means of a free radical crosslinking polymerization route of methacrylic acid in the presence of multi-walled carbon nanotubes (MWCNTs). The structural and morphological characterizations revealed that poly(methacrylic acid) networks (PMAA) closely covered the MWCNTs, and a MWCNT-well-dispersed nanohybrid hydrogel was formed. The addition of MWCNTs strikingly improved pH response and mechanical properties, depending on the component ratios and particle sizes of MWCNTs as well as crosslinker concentrations. The swelling rate was obviously faster than that of the pure PMAA hydrogel. The hydrophilic nature of polyelectrolytes, the capillarity effect, cation-pi or charge-transfer interaction and hydrogen bonds, as well as a subtle balance among these interactions were adopted to interpret the above swelling behavior. Load transfer to the MWCNTs in the networks played important part in compression mechanical improvements. MTT assays were adopted to evaluate the cytocompatibility of the developed biomaterials. This smart hydrogel is expected to be used as potential candidate for specific biological applications.