TTR (transthyretin), a β-sheet-rich protein, is the precursor protein of familial amyloidotic polyneuropathy and senile systemic amyloidosis. Although it has been widely accepted that protein misfolding of the monomeric form of TTR is a rate-limiting step for amyloid formation, no effective therapy targeting this misfolding step is available. In the present study, we focused on CyDs (cyclodextrins), cyclic oligosaccharides composed of glucose units, and reported the inhibitory effect of CyDs on TTR amyloid formation. Of various branched β-CyDs, GUG-β-CyD [6-O-α-(4-O-α-D-glucuronyl)-D-glucosyl-β-CyD] showed potent inhibition of TTR amyloid formation. Far-UV CD spectra analysis showed that GUG-β-CyD reduced the conformational change of TTR in the process of amyloid formation. In addition, tryptophan fluorescence and 1H-NMR spectroscopy analyses indicated that GUG-β-CyD stabilized the TTR conformation via interaction with the hydrophobic amino acids of TTR, especially tryptophan. Moreover, GUG-β-CyD exerted its inhibitory effect by reducing TTR deposition in transgenic rats possessing a human variant TTR gene in vivo. Collectively, these results indicate that GUG-β-CyD may inhibit TTR misfolding by stabilizing its conformation, which, in turn, suppresses TTR amyloid formation.