The archdiocese wants to sell the complex to Perry for $14.5m.
The nuns who lived there agreed to sell it to a businesswoman instead.
A Los Angeles judge has ruled that the sale to entrepreneur Dana Hollister was invalid - but warned that it could take months or years to resolve the dispute.
Los Angeles Superior Court judge James Chalfant deferred a decision on whether Archbishop Jose H Gomez has the right to sell the secluded hilltop property to Perry.
"You're not selling to Katy Perry any time soon," the judge told lawyers for the Archdiocese of Los Angeles.
The Sisters of the Most Holy and Immaculate Heart of the Blessed Virgin Mary have owned the convent for more than four decades, but have not lived there for several years.
It was once home to more than 100 nuns, but only five sisters, aged between 77 and 88, are still alive.
At least two of the five have objected to selling the property to Perry, who is known for her raunchy image and hit songs like I Kissed A Girl.
Sister Rita Callanan recently told the Los Angeles Times she looked up the star online after meeting her. "I found her videos," she said. "I wasn't happy with any of it."
Instead, the nuns agreed to sell the complex to Hollister, who wants to turn it into a boutique hotel.
However, the archdiocese challenged the sale, claiming it had the final say over the fate of the property and that Hollister was taking advantage of the nuns.
On Thursday, the judge said: "There is no doubt in my mind the sale to defendant Hollister was improper and invalid."
But he stopped short of ruling that the archdiocese could sell it to Perry.
The star has reportedly told the nuns she hopes to live on the property with her mother and grandmother.
The judge indicated that the legal case could last for two years. The next hearing will take place on 15 September.