A fibrous plugging device for use in a subterranean well can include a body having fibers and a retainer material. The retainer material retains a shape of the body. A system for use with a well can include a fibrous plugging device conveyed by fluid flow in the well. The fibrous plugging device includes a body comprising multiple fibers. The fibrous plugging device engages an opening in the well, and thereby seals off the opening. A method for use with a well can include flowing a fluid through the well, thereby conveying at least one fibrous plugging device including a body having multiple fibers, and the fibrous plugging device sealingly engaging an opening in the well, thereby substantially preventing flow through the opening.