The mechanism and the origin of the regioselectivity of the rhodium/diene‐catalyzed ring‐opening reactions of C1‐substituted oxabenzonorbornadienes with arylboronic acids has been examined using hybrid density functional theory (DFT). The overall mechanism for the reaction is considered as a four‐step reaction including transmetalation, carborhodation, β‐oxygen elimination, and protonolysis. The free energy profiles show that the rate‐determining step is involved in the protonolysis step which differs from previous mechanistic studies on metal‐catalyzed ring‐opening reactions of oxabenzonorbornadienes. Additionally, it is theorized the β‐oxygen elimination step is the origin of regioselectivity. DFT calculations predict the formation of the 1,2,4‐trisubstituted naphthalene framework is selective over the 1,1,2‐trisubstituted product, in agreement with experimental observations. [ABSTRACT FROM AUTHOR]