The Hills star, 27, announced the happy news on her official website today (Oct 13).
She said: "I am very excited to share with you guys that William and I got engaged over the weekend. I am beyond thrilled! Get ready for lots more wedding content here on LaurenConrad.com as we begin the planning process."
Alongside the announcement, she posted a picture of the huge diamond ring now dominating her left hand!
She later updated the website to thank her fans, saying: "William and I are overwhelmed by all of your warm wishes. Thank you so much for sharing in this special moment :-). XO Lauren."
The reality TV star first started dating musician William, 33, on Valentine's Day 2012, and recently moved him in to her LA apartment.
Lauren's ex-best friend Heidi Montag - who famously fell out with each other on The Hills - was one of the first to congratulate Lauren by tweeting: "Congratulations on your engagement!!! so happy you found true love! I pray you have a long, happy, blessed marriage! xoxo!"
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Copyright: Instagram Mark Wright