The inherently fragile nature of ultrathin polymer films presents difficulties to the measurement of their mechanical properties, which are of interest in packaging, electronics, separations, and other manufacturing fields. More fundamentally, the direct measurement of ultrathin film mechanical properties is necessary for understanding changes in intrinsic material properties at reduced size scales, for example, when the film thickness alters the equilibrium configuration of the polymer chains. We introduce a method for ultrathin film tensile testing that stretches a two-dimensionally macroscopic, yet nanoscopically thin, polymer film on the surface of water. For polystyrene films, we observe a precipitous decrease in mechanical properties (Young's modulus, strain at failure, and nominal stress at failure) for film thicknesses down to 15 nm, less than the characteristic size of an individual polymer chain, yielding new insights into the changes in polymer chain entanglements in confined states.