Next time you see a rat on the subway, tell it to say cheese!

The MTA’s largest union is holding a reverse beauty contest to find the ugliest rat in the subways, part of a campaign to highlight the rodent takeover of the system.

Straphangers looking for the next furry thing should submit their photos to ratfreesubways.com.

The Transport Workers Union Local 100 will then upload the pics onto the site to be ranked by the public. The winner gets a free monthly MetroCard on Jan. 15.