Plasma membrane transporter mediating the uptake by cells of the water soluble vitamin B2/riboflavin that plays a key role in biochemical oxidation-reduction reactions of the carbohydrate, lipid, and amino acid metabolism. May also act as a receptor for 4-hydroxybutyrate. riboflavin(in) = riboflavin(out) Riboflavin transport is Na(+)-independent but moderately pH-sensitive (By similarity). Activity is strongly inhibited by riboflavin analogs, such as lumiflavin (By similarity). Weakly inhibited by flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) (By similarity). Highly expressed in the placenta and small intestine, moderately in the kidney, colon, lung, prostate, uterus, and thymus, and weakly in all other tissues. Belongs to the riboflavin transporter family.