The best-known property of superfluid helium is the vanishing viscosity that objects experience while moving through the liquid with speeds below the so-called critical Landau velocity. This critical velocity is generally considered a macroscopic property as it is related to the collective excitations of the helium atoms in the liquid. In the present work we determine to what extent this concept can still be applied to nanometer-scale, finite size helium systems. To this end, atoms and molecules embedded in helium nanodroplets of various sizes are accelerated out of the droplets by means of optical excitation, and the speed distributions of the ejected particles are determined. The measurements reveal the existence of a critical velocity in these systems, even for nanodroplets consisting of only a thousand helium atoms. Accompanying theoretical simulations based on a time-dependent density functional description of the helium confirm and further elucidate this experimental finding. DOI: 10.1103/PhysRevLett.111.153002 PACS numbers: 33.80.Àb, 36.40.Àc, 67.25.dw Analogous to superconductivity, superfluidity is a macroscopic manifestation of quantum mechanics. It derives its name from the frictionless flow of a liquid [1,2]. While superfluidity has been observed for Bose-Einstein condensates [3] and more recently for polaritons, [4] helium is undoubtedly the best-known example of a superfluid. The peculiar dispersion curve of He dictates that an object moving through superfluid helium can only create elementary excitations if its speed exceeds the so-called critical Landau velocity of $58 m=s [5,6]. Whereas the critical Landau velocity could be experimentally verified in bulk helium, [7] its manifestation in finite size helium systems is still matter of debate [8][9][10]. Knowledge of such a fundamental property becomes essential as finite size helium systems, in the form of helium nanodroplets, are increasingly being used as a matrix for a wide variety of studies [11][12][13].Many properties of helium nanodroplets have been characterized during the last two decades using solvated molecules as spectroscopic probes [14]. Vibrational spectroscopy of solvated carbonyl sulfide (OCS) has provided evidence for microscopic superfluidity in these finite size systems [15]. While a clearly resolved rotational structure was observed in the IR absorption spectrum of OCS in 4 He droplets, this structure was markedly absent in 3 He droplets, which are not superfluid due to their fermionic character. In contrast, the temporal evolution of rotational wave packets of methyliodide molecules dissolved in helium droplets has recently been found to differ dramatically from that of isolated molecules [16]. This raises the question to what extent microscopic superfluidity can be related to the frictionless flow of superfluid helium. Here, we present an approach that uses the translational motion of electronically excited atoms and molecules to probe superfluidity in helium nanodroplets and to establish the existence of a critical velo...