The 37-year-old actress was shocked to find the 47-year-old actor laughing and joking with her relative at a screening of their new movie 'Interstellar' when her gran got so "star-struck" by Matthew and frantically started taking pictures of the hunky Academy Award-winner.
Speaking on an episode of 'Conan', which airs tonight (13.11.14), she said: "He flirted with her like I have never seen! They met at a screening of 'Interstellar' and my grandmother was so star-struck, and she kept saying, 'I want a selfie Matthew!'
"She then proceeded to do a photoshoot in front of everyone, she was like, 'You stand there,' and she just started taking pictures of Matthew.
"But then I took a picture of the two of them, and my God ... I don't think she even realised I was taking the picture."
But Matthew - who has been married to Camila Alves since 2012 - isn't the only star Jessica's grandmother has her eyes on as the flame-haired star explained 'Scarface' actor Al Pacino also caught her eye.
When asked if her family get star-struck around other celebrities, she replied: "I took my grandma, my very first big job was working with Al Pacino, and she took a picture with him and she framed it and put it next to her bed, so it was on her bedside table like ... Al Pacino."
Watch Jessica's interview on 'Conan', tonight at 11pm on truTV, freeview channel 68.