After his hearing, El-Amin said he is resigning from the legislature, the St. Louis Post-Dispatch reported.
El-Amin is the third Missouri Democratic legislator in a month to enter a guilty plea in federal court and quit. State Sen. Jeff Smith and Rep. Steve Brown appeared in court in late August.
The charges against El-Amin were unrelated to those against Smith and Brown.
El-Amin admitted accepting a bribe from a gas station owner.
El-Amin waived indictment. Sentencing is scheduled for Dec. 14.
While El-Amin, a former auto worker, had been in the legislature only since 2006, he had become known for sponsoring legislation that brought him publicity. He introduced a bill to regulate sales of baking soda, which is used in making crack cocaine.