Mayor Bill de Blasio took in a Mets game Sunday, watching his beloved Red Sox lose 5-4 from upper-deck seats at Citi Field.

The Massachusetts native and longtime Boston supporter posed with fans and even City Council members on the stadium's uppermost 500 level.

De Blasio was also at Citi Field on Friday with first lady Chirlane McCray when the Red Sox beat the Mets. The Sox won again Saturday.

City Hall said Sunday that the mayor, a dedicated baseball fan, wasn't necessarily disappointed his team couldn't finish the series with a sweep of the Mets.

"Like many Red Sox fans right now, the mayor wants to see the Mets keep winning," a mayoral spokesman said.

Boston is in last place in their division. The Mets, by contrast, are leading their division.

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City Council members Donovan Richards and Jimmy Van Bramer, both Democrats from Queens, separately tweeted photos of themselves with de Blasio at the ballpark.

De Blasio, also a Democrat, appeared to be wearing shirt sleeves, donning neither Sox nor Mets gear.

"Just another day at the ballpark with my sister Kim and our Mayor @Billdeblasio. let's go mets!" Van Bramer wrote.

De Blasio attended the home opener at Citi Field in April and wore a Mets hat and jersey for the ceremony. He was loudly booed. He said later that all elected officials can expect to be jeered by baseball fans.

The mayor threw out the first pitch at the Mets' 2014 opening day.

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He has so far stayed away from Yankee Stadium.