Navratilova exchanged vows with Toni Layton in New Hampshire in 2000, but the union was not official because gay marriage will not be made legal in the state until 1 January, 2010.
The pair, which started dating in the early 1990s, split its time between houses in Colorado and Florida, but Layton alleges she was left broke and homeless when Navratilova abruptly ended the relationship and booted her out of the home they shared.
According to a lawsuit recently filed by Layton in Sarasota, Florida, the former couple agreed to "evenly share... all assets earned and obtained by either".
Layton claims Navratilova broke the agreement when she cancelled her ex's credit cards and kept all the money from their joint accounts, reports the National Enquirer.
Layton's attorney Raymond Rafool tells the tabloid, "It was a complete shock when Martina terminated the relationship. For eight years Toni was Martina's business partner and domestic partner, but she found herself being manhandled out of the house by Martina's security guards. She was left homeless and practically penniless."
Layton is seeking compensation amounting to half the value of their shared homes, in addition to the return of her three cars and all gifts she gave to Navratilova during their relationship, estimated to be worth $1.5 million (£1 million).
The sports legend's attorney Barry Greenberg has confirmed the legal action but insists the suit is a "private matter", adding, "We're not going to be making any statements."
Meanwhile, the National Enquirer claims Navratilova, 53, has moved on and is planning to wed former Miss Universe beauty contestant Julia Lemigova, 36.