BEYONCE & KELLY ROWLAND Reach Out to Michelle ... WE HAVE YOUR BACK!!

Beyonce and Kelly Rowland immediately dropped what they were doing when they heard Michelle Williams checked into a mental health facility to treat her depression ... and, like good, longtime friends, they reached out to offer their support.

Sources close to Destiny's Child tell TMZ ... Bey and Kelly were incredibly sad after learning Michelle had been struggling with depression and was having such a hard time dealing with it she sought professional help.

We're told both Beyonce and Michelle offered many words of encouragement, expressing their love for her and letting her know -- if Michelle needed anything, they were there to help.