State legislators better watch their back: Bronx Assemblyman Nelson Castro has been wearing a wire.

In a bombshell indictment handed down by the feds, it emerged that Castro — who resigned yesterday — was a key witness in the sting operation against fellow Bronx Assemblyman Eric Stevenson.

Castro, elected to the Assembly in 2008, turned government witness while the state charged him in 2009 with a sealed perjury indictment connected to false statements he made regarding the large number of people who registered to vote from his Bronx apartment, sources said. . .