  We present the first estimates of isothermal compressibility (\kT) of
hadronic matter formed in relativistic nuclear collisions ($\sqrt{s_{\rm NN}} =
7.7$ GeV to 2.76~TeV) using experimentally observed quantities. \kT~is related
to the fluctuation in particle multiplicity, temperature, and volume of the
system formed in the collisions. Multiplicity fluctuations are obtained from
the event-by-event distributions of charged particle multiplicities in narrow
centrality bins. The dynamical components of the fluctuations are extracted by
removing the contributions to the fluctuations from the number of participating
nucleons. From the available experimental data, a constant value of \kT~has
been observed as a function of collision energy. The results are compared with
calculations from UrQMD, AMPT, and EPOS event generators, and estimations of
\kT~are made for Pb-Pb collisions at the CERN Large Hadron Collider. A hadron
resonance gas (HRG) model has been used to calculate \kT~as a function of
collision energy. Our results show a decrease in \kT~at low collision energies
to \sNN~$\sim$~20~GeV, beyond which the \kT~values remain almost constant.
