Gov. Ed Rendell said Stetler would be charged with conduct related to his service as a state representative.
Attorney General Tom Corbett's office did not reveal who would be charged, and Stetler's lawyer did not return phone messages left by The Associated Press.
"The indictment has nothing to do with anything he did while serving in the administration," Rendell told reporters at a news conference on another topic. "That service has been effective and honorable in every way."
Corbett planned an afternoon news conference to disclose additional arrests in the three-year investigation into the Legislature's campaign and financial practices that has already resulted in charges against 22 people.
Stetler represented a York district from 1991 to 2006 and had been a leading campaign strategist for the House Democrats from 2003 to 2006. Rendell appointed him revenue secretary last year.
Rendell disclosed nearly two weeks ago that Stetler had been asked to testify before an investigative grand jury.
Several newspapers reported earlier this month that House Majority Whip Bill DeWeese, D-Greene, also received a grand jury letter asking him to testify. The Pittsburgh Post-Gazette reported that House Majority Leader Todd Eachus, D-Luzerne, also received such a letter. The newspapers did not reveal the sources of the information.
As he headed to the House chamber for a voting session Tuesday, Eachus deflected reporters' questions.
"I know you guys are all going to try to continue to talk to me about this, but I've managed this investigation from the leader's office all year, had no comment all year and have no comment now," Eachus said.
DeWeese's spokesman referred questions to his lawyer, who did not return phone messages Tuesday.
Corbett has previously charged 12 people associated with the House Democratic caucus and 10 people with ties to the House Republicans, including Philadelphia Rep. John M. Perzel, the former House speaker.
In the only case to go to trial so far, former state Democratic Rep. Sean Ramaley of Beaver County was acquitted of all charges last week.
Trial for some of the remaining House Democratic defendants is scheduled to begin Jan. 19, although at least five of them have signed plea agreements with prosecutors. A preliminary hearing for the Republican-linked defendants is not expected until March.