Sydney Sweeney More Swoonin' Over Shirtless Glen ... Won't Stop Flirting!!!

Sydney Sweeney is at it again ... she just can't seem to help but flirt with Glen Powell online -- with his ass-baring magazine photo shoot very much still top of mind.

After flirtatiously claiming her costar's near-nude images were contributing to rising heat levels this winter, Syd showed off yet another personal glimpse of her hunky costar as he worked out onboard a yacht during their film shoot together ... which she threw up on IG.

Check it out ... the clip shows SS recording Glen while they're out on the water during a scene, and it looks like GP was getting a quick workout in between shots -- something Syd was filming at the time, and which we're just now seeing.

She writes, "How @glenpowell actually trained for his men's health cover shoot" ... this while giggling like a school girl and adding a tongue-out emoji for good measure. Mind you, this follows a cheeky comment she left on Glen's naked Men's Health pic earlier this week.

Luckily for Syd, this isn't the first time she's got up close with Glen's buff bod ... as seen in the brand-spankin' new trailer for their rom-com, "Anyone But You," there's a VERY steamy, naked shower scene they acted in together. Just more evidence of chemistry between 'em.

Of course, the pair has been dismissing romance rumors for a while now ... with Glen lamenting to fitness bible MH that the speculation has been "disorienting and unfair."

Meanwhile, Sydney gushed to Women's Health that she wouldn't forget working with Glen and hoped viewers would "feel the love and fun we all shared making this film," ... which is already as clear as day.

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