Chernykh is a Russian citizen, and prosecutors said she married Marquez to obtain legal U.S. residency, even though the two did not live together and never actually had a marriage ceremony.
Syed Raheel Farook and his wife, Tatiana, helped the couple plan and execute the fraud, prosecutors said.
The case against the elder Farook, his wife and sister-in-law, was not related to the Dec. 2 shootings. However, authorities learned of the marriage scheme in the aftermath of the attack.
Farook, who is free on bail, is scheduled to be sentenced in November.
Farook faces a maximum of five years in federal prison under the charge. His attorney, Ron Cordova, declined to comment on why Farook agreed to plead guilty.