The former owner and chief executive of a Houston electronics firm on Wednesday admitted to being a Russian agent and illegally exporting sensitive microelectronics to Russia from Texas, two weeks before he was set to go on trial in Brooklyn, N.Y., federal court.

Alexander Fishenko, 49 years old, was accused of laundering money, violating export laws and obstructing justice. He was also charged with acting as an unregistered agent of the Russian government, a charge that prosecutors say is rarely used and carries a maximum...