Reese Witherspoon showed up in court Friday addressing her father's alleged illegal second marriage and mental incapacity ... asking a Tennessee judge to place him under a conservatorship STAT.
Reese's mother filed a lawsuit earlier this week ... claiming the Oscar-winner's father illegally married ANOTHER woman earlier this year, but was not mentally stable enough to remember ... citing his major drinking problems and on-set dementia in the suit.
Witherspoon's parents have been separated since 1996 ... but they never filed for divorce so legally they are still married.
Reese appeared in front of a probate judge Friday in an emergency hearing and asked that her father be placed under a conservatorship ... claiming he is not capable of making his own decisions.
Witherspoon's mother also obtained a restraining order against her husband's new wife and barred her from using the last name Witherspoon.