Fits to determine parton distribution functions using top-antitop, inclusive $W/Z$ boson, as well as $W^{\pm}$ boson production measurements in association with jets from ATLAS, in combination with deep-inelastic scattering data from HERA, are presented. The ATLAS $W/Z$ boson data exhibit sensitivity to the valence quark distributions and the light quark sea composition, whereas the top-quark pair production data have sensitivity to the gluon distribution. The impact of the these data is increased by fitting several distributions simultaneously, with the full information on the systematic and statistical correlations between data points. The parton distribution functions extracted using $W^{\pm}$ + jets data show an improved determination of the high-$x$ sea-quark densities, while confirming the unsuppressed strange-quark density at lower $x$ < 0.02 found by previous ATLAS analyses.