The pop icon made a series of alarming claims as she asked a judge to end the legal arrangement that controls many aspects of her life in Los Angeles on Wednesday.
Britney said the conservatorship is preventing her from marrying boyfriend Sam Asghari and starting a family with him, as she is being forced to be on birth control.
Following the bombshell hearing, Justin – who had a high-profile relationship with Britney in the late 90s and early 2000s – said what is happening to his former girlfriend is “just not right”.
He tweeted: “After what we saw today, we should all be supporting Britney at this time.
“Regardless of our past, good and bad, and no matter how long ago it was… what’s happening to her is just not right. No woman should ever be restricted from making decisions about her own body.”
He continued: “No one should EVER be held against their will… or ever have to ask permission to access everything they’ve worked so hard for.”
Justin, who split from Britney in 2002, said he and wife Jessica Biel fully support her.
“Jess and I send our love, and our absolute support to Britney during this time,” he said. “We hope the courts, and her family make this right and let her live however she wants to live.”
In February, Justin issued a public apology to Britney after he faced renewed scrutiny following the release of a documentary examining the conservatorship, which she was placed under in 2008.
Freeing Britney Spears looked at how Britney was vilified in the media after their split, and features an old interview where Justin indicated that he’d intentionally chosen a Britney lookalike as a cheating girlfriend in his video for Cry Me A River.
In the apology, which was also addressed to Janet Jackson over their 2004 Super Bowl performance, he said: “I have not been perfect in navigating all of this throughout my career.
“I know this apology is a first step and doesn’t absolve the past. I want to take accountability for my own missteps in all of this as well as be part of a world that uplifts and supports.
“I care deeply about the wellbeing of the people I love and have loved. I can do better and I will do better.”