ET reached out to Dunst and Plemons' reps. Neither responded to emails for comment.
Although some reports claim the 28-year-old actor proposed to Dunst over Golden Globes weekend, it appears the exciting moment may have actually happened earlier. The blond beauty was sporting a stunning sparkler -- an oval-shaped diamond set on a gold band -- on that finger at the Palm Springs Film Festival on Jan. 2.
She wore the same rock three days later during her appearance on The Late Late Show With James Corden.
This will be the first marriage for both Dunst and Plemons, who played married high school sweethearts Peggy and Ed Blumquist on season two of Fargo. The two kept their off-screen romance low-key (never publicly speaking on their relationship status), albeit a PDA-filled date night in Studio City, California, last May.