US magazine InTouch is reporting that the 41-year-old was treated at the Passages Malibu rehab clinic for 30 days.
A source told the magazine that Ellen visited her wife every weekend to provide support.
The pair have made no comment surrounding the rumours.
Actors including Mel Gibson and David Hasselhoff have also been treated at the famous clinic, which claims to be the “world’s most luxurious addiction rehab” centre.
Portia previously sought treatment for an eating disorder in 2000 after her weight reportedly dropped to less than 40 kilograms.
