The mechanical reinforcement of plasticized corn starch (CS) films induced by bentonite nanoclay was investigated using dynamic mechanical analysis and rheology. Dynamic temperature scans revealed mechanical reinforcement from cryogenic to high temperatures, and two mechanical relaxations named α and α’. The lower temperature relaxation α was associated with long range cooperative molecular motions typical of the glass transition and it was associated to glycerin-rich domains, ranging from −60 °C to −40 °C. The mechanical relaxation α′ was detected in the range −30 °C to −10 °C and it may be associated to phase separated starch-rich domains. The morphology was a function of bentonite concentration, at cbentonite<3 wt% the nanoplatelets were exfoliated and c ≥ 3 wt% they were intercalated. Strikingly, at 3 wt% bentonite content the elastic modulus E′ increased drastically. The viscoelastic properties showed that the geometrical confinement exerted by the nanoplatelets also induced a decrease of fractional free volume fg and of apparent molecular weight between crosslinks Mc,app, and an increase of the activation energy Ea of the α’ relaxation. Spatial confinement in the intercalated morphology appears to be the source of mechanical reinforcement of CS/bentonite nanocomposites.