The pseudorapidity distributions of dijets as functions of their average transverse momentum (${p}_{\mathrm{T}}^{\text{ave}}$) are measured in proton-lead ($p\mathrm{Pb}$) and proton-proton ($pp$) collisions. The data samples were collected by the CMS experiment at the CERN LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. A significant modification of the $p\mathrm{Pb}$ spectra with respect to the $pp$ spectra is observed in all ${p}_{\mathrm{T}}^{\text{ave}}$ intervals investigated. The ratios of the $p\mathrm{Pb}$ and $pp$ distributions are compared to next-to-leading order perturbative quantum chromodynamics calculations with unbound nucleon and nuclear parton distribution functions (PDFs). These results give the first evidence that the gluon PDF at large Bjorken $x$ in lead ions is strongly suppressed with respect to the PDF in unbound nucleons.