Jessica Simpson couldn't deny the "Irresistible" urge to get back onstage ... 'cause she just had her first live performance in more than a decade -- and, she says she did it to remember what really inspires her.
The singer-songwriter and actress hit South by Southwest -- the popular film and music festival in Austin, Texas -- performing at the Recording Academy's Austin Chapter Block Party ... and, the event marked her first performance in 15 years.
JS posted a clip before she hit the stage .