Mississippi Gov. Tate Reeves has just signed a bill that will ban transgender athletes from competing in women's sports ... insisting the future law will help keep competition fair.

LGBTQ+ leaders, however, say the governor's actions Thursday could cause serious problems ... including the encouragement of bullying and discrimination.

The bill is titled Senate Bill 2536 in Mississippi, and after Reeves' signature this week, it's slated to become law on July 1.

While it can still face a legal challenge in the courts even after Reeves' decision -- the law will force all public schools in the state to "designate its athletic teams or sports according to biological sex."