Justin Bieber Says He's Not a Dad to Be Messed With in NSFW Father's Day Post

Justin Bieber says anyone thinking about testing him or his family better really question that choice ... 'cause he's not a dad who's going to take that treatment lying down.

The singer-songwriter took to Instagram to post about his approach to fatherhood on Father's Day Sunday -- and, he used some colorful language to ward off anyone looking to take a run at him.

Bieber shared a black and white pic which it appears he added a middle finger emoji to ... and, the caption reads, "I’m a dad that’s not to be f***ed with."

Check out Justin's face in the selfie ... it doesn't look like the mug of someone messing around.

Unclear if Bieber was sending another message to the paparazzi ... but, his Instagram post certainly mirrors video that was taken of him going off on photogs in Malibu just last week.

ICYMI, Justin came out of the popular celeb haunt the Soho House Thursday -- and, he told the press he was at his wits' end with all the attention. He told them he has no problem establishing boundaries now 'cause he's a dad ... so, it seems fatherhood's giving him a ton of confidence.

We detailed Justin's financial struggles, relationship difficulties and more in our new documentary "TMZ Investigates: What Happened to Justin Bieber?" -- now streaming on YouTube. So, check it out to hear everything he's dealing with.