The present invention relates to novel salts and crystalline forms of (S)-2-ethylbutyl 2-(((S)-(((2R,3S,4R,5R)-5-(4-aminopyrrolo[2,1-f][1,2,4]triazin-7-yl)-5-cyano-3,4-dihydroxytetrahydrofuran-2-yl)methoxy)(phenoxy)phosphoryl)amino)propanoate having the structure:for use in treating viral infections. One crystalline form of the compound can be characterized by an X-ray powder diffraction (XRPD) pattern having peaks at 22.3°, 16.9°, and 16.2° 2-θ±0.2° 2-θ. In some embodiments, the viral infection is caused by a virus selected from the group consisting of Arenaviridae, Coronaviridae, Filoviridae, Flaviviridae, and Paramyxoviridae.