Helena City Manager Ron Alles plans to step down in June.
His current employment contract will expire on June 30, and he does not plan to request a new one.
Alles sent a letter to Helena's mayor and city commission Thursday afternoon to inform them of his decision.
"Please know that I will continue to serve the City to the best of my ability through the term of my contract. It has been a true honor and pleasure to be a part of our city government and all of the services we deliver to the citizens of Helena," he wrote.
The letter did not explain why Alles is stepping down, and he could not be immediately reached for comment.
Mayor Wilmot Collins said the city manager's announcement came as a surprise Thursday.
"We appreciate Ron and we appreciate the work he's done for Helena and continues to do," Collins said, noting that he plans to meet with the human resources department Monday to discuss the next steps.
Alles accepted the city manager job in September 2009, according to Independent Record archives. Before that, he spent about nine years as Lewis and Clark County's chief administrator, six years as the county's finance director and three years as a city budget analyst.
Alles was named as one of five finalists for the city manager job in Billings in 2017, but he was not selected.
Independent Record Editor Jesse Chaney contributed to this story.