The 26-year-old pop star - who was hospitalised in July after taking an overdose - has taken to her Instagram Stories to mark the achievement, revealing she celebrated by eating a slice of cake, whilst she also showed off a charm emblazoned with the number six.
The note accompanying Demi's cake read: "Happy 6 mo we are so proud of you! "
And Demi - whose relapse in 2018 came after six years of sobriety - captioned the images: "BEST DAY EVER"
Earlier in January, Demi pledged never to take another day for granted after fighting back to health following her near-fatal overdose.
The chart-topping singer posted: "So grateful for the lessons I've learned this year. I will never take another day in life for granted, even the bad ones."
Demi also thanked her fans for supporting her through her recent troubles.
She said: "Thankful for my fans, friends, family and everyone who supported me through this year. God bless."
Prior to Christmas, Demi revealed she feels "sober and grateful to be alive" after her overdose.
The pop star - who spent time in rehab after she was found unconscious at her Los Angeles home - reassured fans she is fine but also asked for some space.
She wrote on Twitter: "I am sober and grateful to be alive and taking care of ME...
"I'm so blessed I get to take this time to be with family, relax, work on my mind, body and soul and come back when I'm ready. I have my fans to thank for that. I'm so grateful, truly. I love you guys so much thank you."