Britney Spears is proving she just enjoys a good beat, no matter who's singing on the track ... 'cause she posted a video twerking to her ex-boyfriend Justin Timberlake's song.

The singer-songwriter shared another of her famous dancing vids to social media ... wearing a tiny lingerie outfit that let her whole backside hang out.

She starts off by twerking in front of a fireplace before she struts to the camera and starts twisting and turning while staring straight ahead.

Worth noting ... Britney's dance moves aren't the only thing heating up the room ... 'cause there's a fire in the fireplace too -- and, she gets very close to the flames.

Watch the video to see all of her dance moves ... but, more interesting than just her dance is the song she's shaking it to -- Timberlake's "Señorita."

The hit 2003 song -- a collab between Justin and Pharrell Williams -- was co-written by Justin. No one's saying his relationship with Britney influenced the track ... but, she's clearly vibing with it these days.

As you know ... Britney and Justin broke up in 2002 after three years together -- and, in her book "The Woman in Me" Britney claimed he pushed her to get an abortion while they were together and cheated on her.

Justin later issued a faux apology ... which he instantly took back onstage before playing "Cry Me A River" -- a track all about his relationship with Spears.