Justin Long has written a romantic open letter to Kate Bosworth to mark her 40th birthday.

The 44-year-old 'He’s Just Not That Into You' star got candid in a sweet Instagram post dedicated to his girlfriend as she reached the milestone age on January 2 - calling her his "joy" and his best friend as well as listing all her best qualities.

Justin started the note by writing: "She is 40! ... She is also the best part of my day, everyday – even when we’re not together. She laughs with abandon. She makes me laugh the same – all the time ...

"She thanks everyone – even when she’s suffering, she’s kind. She is deeply compassionate and can’t hurt anyone but she can be a rascal. She loves to tease."

Justin - who went public with the romance last year - added they share an interest in "beer and football" and have a lot of fun together.

He went on: "She loves beer and football more than I do (and I love beer and football). She is the strongest person I’ve ever known She’s fiercely committed to the truth. She’s so fun to work with. She’s the most fun to do everything with. She is genuinely curious about people and the world. She makes everything in my life better – all my favourite songs and movies, vacations and sunsets and Chinese food."

He added: "She is the most beautiful human being I’ve ever seen. She is purely good. She will stand up to anyone who isn’t. She has true integrity and grit. She has, by far, my favourite smile .. "

Before concluding his post by writing: "She is my joy. She is my best friend. Happy Birthday HANSAAAY."

In his post Justin also noted that Kate was working on an Instagram message of her own and it turned out she posted a sweet tribute to him to thank him for making her birthday so memorable.

Kate wrote: "You are my love and my light, my peace and my thrill. Thank you for holding my hand when we sleep. Thank you for tying my shoelaces when you notice they are undone. Thank you for making me laugh so hard I have to leave the room. Thank you for showing me what love is and for making this the best birthday ever."