Guanine quadruplexes (G4s) are an important structure of nucleic acids (DNA and RNA) with roles in several cellular processes. RNA G4s require specialized unwinding enzymes， of which only two have been previously identified. We describe the results of a simple and specific mass spectrometry guided method used to screen HEK293T cell lysate for G4 binding proteins. From these results， we validated the RNA helicase protein DDX21. DDX21 is an established RNA helicase， but has not yet been validated as a G4 binding protein. Through biochemical techniques， we confirm that DDX21-quadruplex RNA interactions are direct and mediated via a site of interaction at the C-terminus of the protein. Furthermore， through monitoring changes in nuclease sensitivity we show that DDX21 can unwind RNA G4. Finally， as proof of principle， we demonstrate the ability of DDX21 to suppress the expression of a protein with G4s in the 3΄ UTR of its mRNA.