Reese Witherspoon's sweet tribute to son Deacon as he turns 20: 'I love you!'

Reese Witherspoon shared a sweet birthday tribute to her lookalike son Deacon on Tuesday, as he celebrated his 20th birthday.

The Legally Blonde actress, 47, shared a series of pictures of herself with Deacon as she marked the milestone birthday.

"HUGE Birthday wishes to my boy @deaconphillippe!!" her post began.

Watch the video above.

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"Actually not a boy, he is a 20 yr old man today ! 🥰 Deacon, you are such a ray of sunshine, love and positivity in this world."

She added: "Keep shining that wonderful light on us all! I 💗you!"

Underneath the post, Deacon commented: "Love u mama."

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In her post, she shared a picture of Deacon smiling while patting a pet pooch and in another, he poses with a hat and scarf on as she poses for a selfie.

She also shared a video of the aspiring performer singing on stage.

Also sharing a tribute to Deacon was his sister Ava, 24, who wrote: "What a rare gem you are. Smart as you are kind, fun-loving as you are loyal, hard-working as you are handsome…"

"Those of us fortunate enough to know & love you benefit from how generously you share your positivity, compassion, & creativity, and I hope you recognise the tremendous impact you've already had on so many of the people in your life."

She added: "Today begins your next decade of dreaming big & living life to the fullest, and I could not be more thrilled to cheer you on through it all.♥️Aves."

Witherspoon shares Ava and Deacon with her ex-husband, actor Ryan Phillippe, 49.

The couple, who starred in the 1999 hit Cruel Intentions together, split in 2008.

They tied the knot three months after the film came out, Witherspoon was 23.

"I was so, so young, I was, like, ridiculously young," Witherspoon said in a previous interview with Elle.

"I learnt a lot, though. It was an excellent opportunity for me to really find out what it means to be a partner and to be in a marriage and it's not anything that I thought it was.

"You can't say, 'I don't want to talk about that'. You just can't. You've got to talk about it all."

She went on to marry and welcome a child with husband Jim Toth, but she filed for divorce in March after 12 years of marriage.