Thin-film membranes consisting of nanoparticles are of interest in applications ranging from nanosieves to electric, magnetic, or photonic devices and sensors. However, the fabrication of large-scale membranes in a simple but controlled way has remained a challenge, due to the limited understanding of their mechanical properties. Systematic experiments on ultrathin, freestanding nanoparticle membranes of different core materials, core sizes, and capping ligands are reported. The results demonstrate that a drying-mediated self-assembly process can be used to create close-packed monolayer membranes that span holes tens of micrometers in diameter. Containing up to ≈107 particles, these freely suspended layers exhibit remarkable mechanical properties with Young's moduli of the order of several GPa, independent of membrane size. Comparison of three different core–ligand combinations suggests that the membrane's elastic response is set by how tightly the ligands are bound to the particle cores and by the ligand–ligand interactions.