Using data from the NEMO-3 experiment, we have measured the two-neutrino double beta decay ( 2νββ ) half-life of82 Se as T1/22ν=[9.39±0.17( stat )±0.58( syst )]×1019 y under the single-state dominance hypothesis for this nuclear transition. The corresponding nuclear matrix element is M2ν=0.0498±0.0016 . In addition, a search for neutrinoless double beta decay ( 0νββ ) using 0.93 kg of82 Se observed for a total of 5.25 y has been conducted and no evidence for a signal has been found. The resulting half-life limit of T1/20ν>2.5×1023 y (90% C.L. ) for the light neutrino exchange mechanism leads to a constraint on the effective Majorana neutrino mass of ⟨mν⟩<(1.2−3.0) eV , where the range reflects 0νββ nuclear matrix element values from different calculations. Furthermore, constraints on lepton number violating parameters for other 0νββ mechanisms, such as right-handed currents, majoron emission and R-parity violating supersymmetry modes have been set