The controversy surrounding his 2021 release, Something for Thi Hotties, was classified as a mixtape.
It debuted at number three on the US Billboard Top R&B/Hip-Hop Albums chart in its first week of release.
In the lawsuit filed Friday, Megan's team says the release "clearly meets the definition of an "album."
In the document seen by Radio 1 Newsbeat, it says that 1501 seeks to "turn [Megan] down" for his "financial gain".
Her legal team believes that the label wants to keep her locked in their contract long enough so that they can profit from her sale.
Record deals are usually signed with an agreement on how many albums the artist will release before they can renegotiate or leave the label.
Public quarrel
This isn't the first time Megan has had a dispute with her record label.
In 2020, she launched a previous lawsuit against 1501, claiming they were preventing her from releasing new music because she wanted to renegotiate the terms of her contract.
On Instagram Live at the time, she said: "When I signed, I didn't know what was in my contract.
"I was younger. I think I was 20, and I didn't know everything that was in my contract."
It has since been dropped, and the label allowed her to release the music.
The 1501 boss, Carl Crawford, posted on Instagram on Tuesday, citing a lawsuit that has been resolved since 2020.
Megan shared a screenshot of her post on her Instagram - which she has since deleted. She said that Crawford "never knows [what] is going on with the business."
"We're definitely still in court and you're still suing bc you owe me money!
"I've never been paid in my life since 1501!"
The new lawsuit is not seeking payment from 1501 except for legal fees, only the recognition that Something for the Hotties is an album.
What is an album?
In the lawsuit filed by Megan's attorneys, it states: "'Something for the Hotties' clearly meets the definition of 'album' under the recording agreement because it is not less than forty-five (45) minutes."
According to lawyers, Megan's contract only required a length of 45 minutes or more to define what an album is.
The lawsuit states: "The new status of 1501, months after the album's release, is clearly a ploy in an attempt to take more advantage of [Megan], not in good faith."
At the 2021 Grammys, Megan took home Best New Artist and, along with Beyoncé, won Best Rap Performance and Best Rap Song for "Savage (Remix)".
Newsbeat has contacted Megan Thee Stallion's management and 1501 for comment, but no one has responded.