Growing evidence supports that amyloid beta (A beta) oligomers are the major causative agents leading to neural cell death in Alzheimer's disease. The polyphenol (-)-epigallocatechin gallate (EGCG) was recently reported to inhibit A beta fibrillization and redirect A beta aggregation into unstructured, off-pathway oligomers. Given the experimental challenge to characterize the structures of A beta/EGCG complexes, we performed extensive atomistic replica exchange molecular dynamics simulations of A beta(1-42) dimer in the present and absence of EGCG in explicit solvent. Our equilibrium A beta dimeric structures free of EGCG are consistent with the collision cross section from ion mobility mass spectrometry and the secondary structure composition from circular dichroisrn experiment. In the presence of EGCG, the A beta structures are characterized by increased inter-center-of-mass distances, reduced interchain and intrachain contacts, reduced beta-sheet content, and increased coil and alpha-helix contents. Analysis of the free energy surfaces reveals that the A beta dimer with EGCG adopts new conformations, affecting therefore its propensity to adopt fibril-prone states. Overall, this study provides, for the first time, insights on the equilibrium structures of A beta(1-42) dimer in explicit aqueous solution and an atomic picture of the EGCG-mediated conformational change on A beta dimer.