The Barbarella star spotted the urn in her hotel room after a long day at work and presumed the contents were bathing products.
Fonda proceeded to run a bath and pour in the ashes, but she was left aghast after spotting a bone and realising she was actually handling the ashes of her dead pooch Roxy.
She tells Britain's Daily Express, ""One evening, when I came home from work, I found an urn sitting on my hotel room table with a note, 'Until We Meet Again'. I thought, 'What could this be and who's it from?' I opened the urn and it appeared to be bath salts.
""So I ran a nice hot bath and dumped some of the 'salts' into it. But a small fragment of bone fell out as well and I realised, with shock, they were Roxy's ashes.
""I took the bath anyway and it felt good.""