In the funny clip, Swift, wearing her signature Red-era lipstick, mouths the words, "Oh no, my bestie is a bad b**ch," before turning the camera to Gomez, sporting a white turtleneck, who smiles and laughs with glee.
"OH NO @selenagomez," Swift captioned the video, which came after the release of Red (Taylor's Version) on Friday.
Reynolds and Lively are longtime friends of Swift, with the singer even using their kids' names in her 2020 song, "Betty."
The couple was also spotted attending the SNL after-party with the singer, who recently announced that Lively will make her directorial debut with Swift's upcoming music video, "I Bet You Think About Me."
Meanwhile, Stranger Things star Sink co-starred alongside Dylan O'Brien in All Too Well: The Short Film, which Swift wrote and directed to accompany the long-awaited 10-minute version of her song, "All Too Well."
When ET's Nischelle Turner spoke to Swift at the premiere of the short film, she praised the actors for their work on the project.
"They just gave everything and I trusted them so much because I watched everything they've done," Swift said. "I've been a fan of theirs for ages, and so I knew-- I had a feeling, if you put these two people in a room and you trust their instincts, magical things could happen."
"They were electric in this," she added. "I really just am so proud of those performances and I'm proud of how it looks and I'm proud of how it feels to watch it."