Many RNA structures are composed of simple secondary structure elements linked by a few critical tertiary interactions. SHAPE chemistry has made interrogation of RNA dynamics at single-nucleotide resolution straightforward. However, de novo identification of nucleotides involved in tertiary interactions remains a challenge. Here we show that nucleotides that form noncanonical or tertiary contacts can be detected by comparing information obtained using two SHAPE reagents, N-methylisatoic anhydride (NMIA) and 1-methyl-6-nitroisatoic anhydride (1M6). Nucleotides that react preferentially with NMIA exhibit slow local nucleotide dynamics and usually adopt the less common C2'-endo ribose conformation. Experiments and first-principles calculations show that 1M6 reacts preferentially with nucleotides in which one face of the nucleobase allows an unhindered stacking interaction with the reagent. Differential SHAPE reactivities were used to detect noncanonical and tertiary interactions in four RNAs with diverse structures and to identify preformed noncanonical interactions in partially folded RNAs. Differential SHAPE reactivity analysis will enable experimentally concise, large-scale identification of tertiary structure elements and ligand binding sites in complex RNAs and in diverse biological environments.