In a California court, Jawbone accused Fitbit of infringing three of its patents, just a few weeks after alleging the company had poached staff and stolen commercially sensitive data.
Fitbit said it would "vigorously defend" itself against the allegations.
It is preparing to list on the New York Stock Exchange later this month, hoping to raise $358m from the share sale.
A Jawbone spokesperson said: "An extensive review revealed that Fitbit's wearables infringed on our patents.
"In order to protect our investment in products and technology, we felt we had no choice but to take this action."
In the legal complaint, Jawbone said that it had spent more than $100m in research and development for its products in the last two years.
The company said that "Fitbit's acts of infringement" had damaged the company and that it was "entitled to recover from Fitbit the damages they have sustained as a result of Fitbit's wrongful acts in an amount subject to proof at trial".
Fitbit said: "As the pioneer and leader in the connected health and fitness market, Fitbit has independently developed and delivered innovative product offerings to empower its customers to lead healthier, more active lives.
"Since its inception, Fitbit has more than 200 issued patents and patent applications in this area. Fitbit plans to vigorously defend itself against these allegations."
The wearable fitness firm Jawbone has filed a second legal complaint against its rival Fitbit.