Trump has made Strzok a frequent target, citing those disparaging texts, and has repeatedly argued that Strzok's political bias tainted the early stages of the Russia investigation.
But in combative testimony before Congress in July of last year, Strzok told lawmakers that, "at no time, in any of these texts, did those personal beliefs ever enter into the realm of any action I took."
When the DOJ's inspector general uncovered the text messages between Strzok and then-FBI lawyer Lisa Page, he was removed by Robert Mueller from the special counsel's office to a lower-level human resources position before his eventual firing from the bureau. Page has left the FBI but also is a frequent Trump target.
One text exchange between the two that especially rankled Trump read: "[Trump's] not ever going to become president, right? Right?!" Page wrote to Strzok in August 2016.
Strzok, responded: "No. No he's not. We'll stop it."
In the lawsuit, Strzok's lawyers argue that his firing came after Assistant Director of the Federal Bureau Investigation Office of Professional Responsibility Candace Will recommended that Strzok not be fired, and instead receive a demotion and suspension for 60 days without pay. Despite Strzok agreeing to the so-called "last-chance agreement," he was still fired from the bureau on August 9, 2018.
"The FBI fired Special Agent Strzok because of his protected political speech in violation of his rights under the First Amendment to the Constitution of the United States," the lawsuit said. "The FBI also deprived Strzok of his property interest in his employment without due process, in violation of his rights under the Fifth Amendment to the Constitution."
The lawsuit calls for Strzok to receive, "equitable and injunctive relief, including reinstatement and back pay ... as well as actual damages for the violations of the Privacy Act."
ABC News reached out to DOJ for a response to the lawsuit, but the department declined to comment.