After a legal battle spanning nearly two years, a judge has ruled in favour of pop star Katy Perry in her bid to buy a Los Angeles convent from a group of nuns.

In June 2015, the singer offered $US14.5 million ($18.9 million) for the property, but the sisters of the Immaculate Heart of Mary opted to sell it to restaurateur Dana Hollister, who had put in an offer weeks later.

The sale was contested in court the same month, because it was in dispute who actually had the right to sell the property in the first place.

Los Angeles Archbishop Jose Gomez argued he had the legal right to sell the property to Perry, rather than the nuns living at the convent.

In a September 2015 court filing, Perry said if she owned the property she would preserve the convent as a "residential oasis" and would let the sisters continue living there for up to two years.

Perry claimed that Hollister was manipulating Sisters Rita Callanan and Catherine Rose Holzman, two of the nuns living at the convent who had blocked Perry's sale.

On March 14 this year, Los Angeles Superior Court Judge Stephanie Bowick ruled the nuns failed to get the necessary written approval for the sale from the LA Archdiocese as well as the Pope. Therefore the sale to Dana Hollister was invalid, and the property is now available for Katy Perry to buy.

"No consent was obtained before the close of the sale," Judge Bowick wrote in the ruling. "The above failures render the Hollister transaction invalid," Mansion Global reported.

"The court finds that the sisters did not have authority to sell the property to Hollister," wrote Judge Bowick in her 34-page ruling.

"Even assuming that the Sisters had the authority to dispose of the property, which they did not, they nevertheless failed to validly consummate the transaction. The deal documents were not properly documented," the judgment read.

Bowick made a similar ruling in April 2016, but at that time the court had not reached a legal conclusion as to whether the transaction, through Perry's company Birds Nest, was valid.

"The main concern of the Archdiocese is and has always been the care and well-being of all the IHM Sisters," the archdiocese said in a statement obtained by The Hollywood Reporter.

"The Archdiocese was forced to take legal action in 2015 on behalf of all the Sisters when developer Dana Hollister took possession of their property without proper authorisation for just $44,000 and a contingent promissory note without any guarantee that the Sisters would ever receive any additional payment," read the statement.

Due to the title of the property, Pope Francis will have to approve its final sale to Perry.