The 24-year-old singer is taking action against Tyler Armes in response to the songwriter's own legal complaint against the star after he claimed he wasn't paid for his contribution to the song.
In court documents obtained by TMZ, Tyler explained he and Post were encouraged to write together by the 'Sunflower' hitmaker's manager in August 2018 and they eventually came up with 'Circles'.
Tyler claimed he co-wrote the chords and bass line for the track, and had input in other areas, such as the guitar melody, so he was shocked when the track was released in the summer of 2019 and he didn't receive a writing credit.
According to the lawsuit, the songwriter had been negotiating with Post's team a few weeks before and they initially offered him a 5 per cent share of publishing royalties but no credit, rejecting his request for a credit and a bigger cut of the money.
Tyler claimed that Post's manager, Dre London, had engaged in a written exchange about the track and even told him that the rapper had acknowledged his contribution.
The songwriter - who is also suing producer Frank Dukes, who was at the jam session and does have a writing credit - has taken action against the 'rockstar' hitmaker in order to receive the credit and a fair cut of past and future royalties on 'Circles'.
In response to the California lawsuit, Post - whose real name is Austin Richard Post - filed his own case in New York, in which he insisted Tyler didn't write any of the music or lyrics at the meeting he was present at, and nor did he attend future recording sessions for the song.
He has asked the court to rule that Tyler is not a co-writer and doesn't deserve money from the song.
His lawsuit stated: "It is an age-old story in the music business that when a song earns the type of runaway success that 'Circles' has garnered, and individuals will come out of the woodwork falsely claim to take credit for the song, and demand unwarranted and unearned windfall profits from the song."