Though the two haven't connected since news of Grande's engagement to Saturday Night Live star Pete Davidson broke, Trainor couldn't help but gush over the couple.
"I'm so happy for them," she tells ET. "They're the cutest thing of all time!"
Trainor says she last saw Grande at Wango Tango, where the "No Tears Left to Cry" singer was wearing her new bling on that finger.
"I saw that! And my mom texted me like, 'Yo, does anyone want to talk about this?'" Trainor admits.
Grande will release her new album, Sweetener, on August 17. Shortly after, Trainor will unveil her own full-length, Treat Myself, on August 31. It appears that both pop stars could be eyeing winter weddings, as well, with a source previously telling ET that Grande "loves the idea of a winter wonderland-type scene" for her wedding day.
At Wednesday's taping of The Four in Studio City, California, Trainor said she's plotting a Christmas-time wedding in her own backyard.
"Nice and cold," she says of the weather, "so that when we dance and get sweaty, we can, like, cool off, you know?"
For more from Trainor, watch the video above. The Four airs Thursdays on Fox.