Lauren Meizo -- a hardcore Patriots fan waiting for her 2nd heart transplant in 5 years -- got a helluva surprise Monday when Rob Gronkowski and his brother, Gordie, visited her in the hospital!

27-year-old Meizo had made a YouTube video saying her wish was to meet the Pats tight end and begged her friends to share the message with anyone who knew Gronk.

Her plan worked ... the video went viral and Gronk answered the call -- rolling up to the Brigham & Women’s Hospital in Massachusetts with his brother Gordie to hang with Lauren!

"Surprised Lauren Meizo w @gronk," Gordie posted on Instagram ... "It was great you allowed us to have a meet and greet with you! #gronksandwich #meizostrong."

FYI, Meizo has been living in the hospital for the past 3 months awaiting her heart transplant and a kidney transplant.

Get well soon!