CHICAGO (Reuters) - Lois Scott, Chicago's chief financial officer, said on Friday she plans to step down this month after four years on the job, making the announcement two days after she helped unveil plans to clean up the debt practices of the third-largest U.S. city.

Mayor Rahm Emanuel's office said Scott will leave on May 20.

"It has been a special honor serving Mayor Emanuel, a visionary mayor who believes deeply that confronting our fiscal challenges is inextricably linked to the larger economic growth and job-creation strategy for Chicago," Scott said in a statement.

In a statement, Emanuel called Scott "an essential member" of his economic team and thanked her for her service. The mayor noted that he had asked Scott to stay in the post for one year, but she showed her commitment to serve by staying for four.

The financially troubled city plans to end interest-rate swaps and phase out bond restructurings in changes expected to cost more than $275 million, Chicago financial officials said on Wednesday.

Emanuel also announced steps on Wednesday to restore "fiscal sanity" to Chicago's sagging budget.

Chicago is struggling with a $20 billion unfunded pension liability and a looming $550 million increase in pension contributions that needs to be made from a budget with a $300 million structural deficit.

(Reporting by Mary Wisniewski; Editing by Will Dunham)