This mom went off the rails.

A woman riding the subway in The Bronx handed her infant off to a random straphanger — so she could go “fight some b- - -h,” according to a witness who caught the drama on cellphone video.

Javier Clemente, 19 — who posted the short clip on Twitter Wednesday — was riding a train on the 2/5 line when the woman handed over her bundled-up baby girl.

“Yo, this is her baby!” a grinning Clemente, who goes by @Icedoutguccifur on Twitter, says in the video, showing off the unfazed child sucking on a pink pacifier.

The camera then pans to a woman holding a phone to her ear and standing in the aisle of the train, and someone can be heard in the background shouting, “I want her to get off because I’m gonna hit her! I’m gonna hit her!”

“She’s talking s- -t . . . She’s gonna hit my mother,” the person can be heard saying.

It was unclear whether any punches were thrown during the incident.

“BRONX LADY GIVE ME HER KID TO HOLD TO FIGHT SOME BITCH,” Clemente tweeted with the video, which has been widely shared on social media.

The clip had been viewed more than 10,200 times on Facebook, where Clemente posted it Tuesday with the request, “SHARE SHARE SHARE !!”

Clemente even posted on his Instagram page a photo of himself smiling with the woman he was with and the baby, geo-tagging the location at 174th Street.

“When you in the Bronx and some lady give you her kid to go fight some b- - -h,” the selfie’s caption read.

The photo garnered dozens of comments, ranging from amused to disturbed reactions.

“And this is what’s wrong with America because . . . mothers like this,” @pachucaasunrise wrote.

@rebirth_xi wrote, “[Very] proud of my borough and our mess.”

“Pic just made my day,” @amani.haze commented.

Another commenter asked, “did she win?”

Commenting on the video on Instagram, user @lxzzg joked, “The baby’s like, ‘yeah, I’m used to this.’ ”

One Twitter user said of the video, “that’s actually incredibly sad.”

A spokesman for the NYPD could not provide any details about the incident.

Clemente did not return requests for comment.