Brittney Griner backed out of a planned speaking engagement this week ... after she found a note in her hotel room that she took as a threat.
TMZ Sports has learned ... the WNBA superstar was slated to talk at the Women Grow Leadership Summit in National Harbor, Md. -- but bolted from the event on Monday morning after discovering a piece of duct tape near her entryway that had the words "Gay Baby Jail" written on it.