Sydney Sweeney Claps Back at Trolls Commenting on Body with Workout Video

Sydney Sweeney's ready to knock out all the haters talking trash from behind a keyboard ... sharing a clip with all their negative comments about her body -- cut together with a powerful workout montage.

Here's the deal ... the actress -- who plays boxing legend Christy Martin in an upcoming biopic -- has been slammed by internet trolls in recent days saying she's gained too much weight and isn't hot enough for them anymore.

Sweeney put many of the body-shamers on blast though by cutting their comment into a video ... flashing through each of them quickly -- before cutting to a clip of the star getting swole.

Check out the clip ... she's flipping tires, sparring in the ring, hitting the speed bag doing squats, crunches -- the whole circuit in the gym really.

She's flexing in the clip too -- and, we gotta say she's looking jacked ... putting on some weight that's all clearly pure muscle.

We've shown you some shots of Sydney on set in the past few months ... and, she's totally transformed herself to embody Martin -- looking exactly like the boxing standout.

She's been spotted on set in baggy red and baggy pink tops with sweatpants ... and brown wigs covering up her light blonde hair.