The president of the Sheet Metal Workers union is resigning.

Joseph J. Nigro, who acted as general president of the International Association of Sheet Metal, Air, Rail, and Transportation Workers since July 2011, will step down April 30, the union announced.

Current General Secretary-Treasurer Joseph Sellers Jr. will become the union’s president, officials said. Staff chief Richard McClees has been named secretary-treasurer.

Nigro, a career sheet metal works employee with Boston-based Local 17, said health issues led to his decision to retire.

“I have committed my heart and mind to accomplishing the goals we have set, but my health has taken me in a direction I did not anticipate,” Nigro said in a letter to union officials. “Unfortunately, I have been unable to recover to a level I expect of myself as your general president — the level of effort the members deserve.”

The 20,000-member union, which goes by the acronym SMART, is the result of a merger between the Sheet Metal Workers and the United Transportation Union.