Check it out ... the artist stepped out in a black leather look arriving at The Nice Guy in Los Angeles Monday to ring in her 29th birthday. Doja kept things entirely dark for her name-day celebration ... including rocking raven hair for the occasion.

Her celebrity friends followed suit with their attire, too. For instance, Hailey Bieber arrived in a long black leather trench coat and itty-bitty black sunglasses ... reminiscent of the get-up worn in "The Matrix."

Hailey's pop star hubby, Justin Bieber, donned his own dark ensemble ... throwing on a fuzzy black jacket, hoodie, baggy pants, tee, and combat boots. Though, Justin appeared to want to keep a low profile at the party ... keeping his head down as he entered the hot spot.

Hours before swinging by Doja's birthday party, JB paid tribute to the late Liam Payne ... who fell to his death from a hotel balcony in Buenos Aires, Argentina Wednesday. So, that could factor into the singer's serious expression Monday night.

The Biebers weren't the only big names in attendance. Camila Cabello and Reggie Watts also rolled into the A-list affair ... wearing dark styles for the evening out.

Camila's ensemble is worth taking a closer look, BTW ... as she left little to the imagination with a cheeky outfit. As she sings ... "My Oh My"!!!