Ronda Rousey is finally owning up to a mistake she made more than a decade ago ... formally apologizing for sharing a Sandy Hook conspiracy theory vid back in 2013.
The former UFC bantamweight champion infamously reposted the clip -- which questioned the authenticity of the tragic school shooting that took the lives of 20 children and six staffers -- to her social media page ... before almost immediately taking it down amid backlash.
She appeared to try to sweep it under the rug in the ensuing years .