John Hinckley, who tried to assassinate US president Ronald Reagan in 1981, regained his freedom fully on Wednesday, six years after he was released from a psychiatric hospital.

Earlier this month a Washington court ruled that after decades of treatment and psychiatric reviews, Hinckley no longer presented a threat, and conditions set on his life after release would be lifted on June 15.

"After 41 years 2 months and 15 days, FREEDOM AT LAST!!!" he tweeted in celebration Thursday.

Hinckley, now 67, shot Reagan and three others with a revolver outside a Washington hotel on March 30, 1981. Hinckley said he wanted to impress actress Jodie Foster, with whom he became obsessed after watching her in the film "Taxi Driver."

All four people he shot survived, although Reagan press secretary James Brady was left partially paralysed and forced to use a wheelchair.

At his trial in 1982, Hinckley was declared not guilty on grounds of insanity, and admitted to St Elizabeth's Hospital, a psychiatric institution in Washington, for 34 years.

He was released in September 2016 but required to live with his elderly mother in a gated community in Williamsburg, Virginia under a long list of restrictions.

Those included controls on his movements and monitoring of his electronic devices and online accounts.

He was also forbidden to contact Foster or travel to any area where a current or former president, vice president or member of Congress would be present.

Nor could Hinckley speak to the media or post any writings or memorabilia on the internet, or display them in person without authorisation.

A government report on his status filed to the court on May 19 said his mental health had "remained stable" and that his psychiatric illness had been in "full and sustained remission for decades." (AFP)