James Holmes -- the monster behind the gruesome "Dark Knight" massacre last year -- has just entered a not guilty plea ... by reason of insanity.
A plea of not guilty had previously been entered on Holmes' behalf back in March by the judge because Holmes' lawyers weren't prepared to plead for him at the time.
The judge will decide whether to accept Holmes' new plea by May 28th.
Holmes killed 12 people and injured an additional 58 during the Aurora, Colorado shooting last July. He is charged with 142 crimes, including 24 1st degree murder counts, and 116 counts of attempted murder.
Prosecutors announced last month they would be seeking the death penalty.