We give an explicit characterization of the standard monomials of positroid varieties with respect to the Hodge degeneration, and give a Gr\"obner basis for positroid varieties. As an application, we show that promotion on rectangular-shaped semistandard tableaux gives a bijection between standard monomials of a positroid variety and its cyclic shifts.PDF: Standard Monomials and Gr\"obner Bases for Positroid Varieties.pdf