The 94-year-old actress - who is bedridden following a string of health problems - was delighted by Prince Frederic von Anhalt's gesture, which saw him take out a huge poster board on Los Angeles' Sunset Boulevard, with the advertisement featuring a picture of the couple on their wedding day from August 14 1986 and a caption reading: "Prince Frederic & Princess Zsa Zsa 25 years and counting."
Explaining Zsa Zsa's reaction, Frederic said: "She said, 'Oscar De La Renta.' It was her favourite dress, and she looked at it and she remembered it."
To mark their anniversary next month, Frederic is planning to throw a party, despite the fact his wife is bedridden.
He told The Huffington Post: "She is better today than she was two months ago.
"I've invited a few people, a few of our old friends - some of them are still alive - I'm going to get them up to the house.
"Hopefully Mrs. Reagan is going to come, and a couple of the young stars too.
"Zsa Zsa will be In The Bedroom, and our closest friends can then go to her in the bedroom and say hi to her. Singularly, so there will be no mess."
The billboard will stay in place for a month.