Mayoral hopeful Eric Adams on Thursday won the support of the MTA’s largest union, Transport Workers Local 100.

The union’s executive board selected Adams by an overwhelming 40-to-3 margin on Thursday afternoon, sources said.

While the MTA is a state-run agency, Adams and TWU share an interest in clamping down on subway crime. The retired cop-turned-elected official recently accused the NYPD of not effectively deploying its officers within the transit system.

The Brooklyn borough president already had significant union support in his bid to be New York City’s 109th mayor.

The powerful building workers union 32BJ, Hotel Trades Council and District Council 37 — which represents the city’s municipal workforce — all back Adams’ candidacy.

TWU Local 100 represents nearly 40,000 subway and bus workers. The union’s board also considered candidates Kathryn Garcia, Scott Stringer, Dianne Morales, Shaun Donovan and Andrew Yang before settling on the retired NYPD cop, sources said.

The group backed second-place finisher Bill Thompson in the 2013 Democratic primary, and has been closely aligned with Gov. Andrew Cuomo.