On Wednesday, the singer told a Los Angeles judge being under an "abusive" conservatorship left her "traumatised".
The arrangement prevents Spears from controlling many aspects of her life.
"After what we saw today, we should all be supporting Britney at this time," said Timberlake, who dated Spears two decades ago.
"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."