The "Only Murders in the Building" actress stepped out for the awards show at Los Angeles' Peacock Theater Sunday evening ... where she wore a striking black Ralph Lauren gown and several noticeable diamond rings -- including a platinum band covered in marquise diamonds on her left ring finger.

Of course, having such a bauble on that finger instantly got tongues waggin' about the possible engagement of the Disney Channel alum and her music producer boyfriend, Benny Blanco.

Not to mention, Selena and Benny were all over each other during the awards show ... giving the camera ample amounts of PDA to capture -- as a possibly engaged couple might do.

Yet, according to E! News, the ring in question was just one of many Tiffany & Co. pieces she chose to wear on the red carpet at the event ... where she was nominated for Outstanding Lead Actress in a Comedy Series.

We can't blame fans for jumping to conclusions, however ... as the blinged-out outing came one month after Selena prompted engagement speculation by covering her ring finger with an emoji in a photo with her beau.

As TMZ previously reported ... Selena then notably followed a popular wedding planner on TikTok before hitting the streets of L.A. wearing a gold band -- again, on her ring finger.

It's unclear if Benny has actually gotten down on one knee this summer ... but the rumors are certainly heatin' up. Where there's smoke there's fire, right???