Significance The cell cytoplasm is a multicomponent solution that is much more concentrated than typically sampled in conventional in vitro studies. The presence of cosolutes can strongly influence biomolecular folding due to “molecular-crowding” effects, which are poorly understood and yet crucial to understand. In this study, the thermodynamics/kinetics of “crowding” effects are probed at the single-molecule level for an isolated RNA tertiary interaction. We observe dramatic stabilization of the folded state and, for the first time (to our knowledge), establish the kinetic origin of molecular crowding to be dominated by entropic acceleration of folding.