The first measurement of the jet mass $m_{\text {jet}}$ of top quark jets produced in ${\mathrm{t}}\overline{\mathrm{t}} $ events from pp collisions at $\sqrt{s}=8$ $,\text {TeV}$ is reported for the jet with the largest transverse momentum $p_{\mathrm{T}}$ in highly boosted hadronic top quark decays. The data sample, collected with the CMS detector, corresponds to an integrated luminosity of 19.7 $,\text {fb}^{-1}$ . The measurement is performed in the lepton+jets channel in which the products of the semileptonic decay ${\mathrm{t}} \rightarrow \mathrm{b} \mathrm {W}$ with $\mathrm {W}\rightarrow \ell \nu $ where $\ell $ is an electron or muon, are used to select ${\mathrm{t}}\overline{\mathrm{t}} $ events with large Lorentz boosts. The products of the fully hadronic decay ${\mathrm{t}} \rightarrow \mathrm{b} \mathrm {W}$ with $\mathrm {W}\rightarrow \mathrm{q} \overline{\mathrm{q}} '$ are reconstructed using a single Cambridge–Aachen jet with distance parameter $R=1.2$ , and $p_{\mathrm{T}} >400$ $,\text {GeV}$ . The ${\mathrm{t}}\overline{\mathrm{t}} $ cross section as a function of $m_{\text {jet}}$ is unfolded at the particle level and is used to test the modelling of highly boosted top quark production. The peak position of the $m_{\text {jet}}$ distribution is sensitive to the top quark mass $m_{{\mathrm{t}}}$ , and the data are used to extract a value of $m_{{\mathrm{t}}}$ to assess this sensitivity.