Legal documents suggest that Sheeran has stolen elements from the 1973 classic, citing reasons for legal action relating to "the melody, rhythms, harmonies, drums, bass line, backing chorus, tempo, syncopation and looping".
Other defendants listed in the new claim include Sony/ATV Music Publishing, the Atlantic record label and Amy Padge - who co-wrote the song. He was also sued almost two years ago by the heirs of the song's co-writer, Ed Townsend, but the suit was later thrown out by a judge in February 2017.
"Thinking Out Loud" peaked at No. 2 on the Billboard Hot 100 and topped United Kingdom charts; the song has also earned Sheeran Grammy awards for song of the year, and best pop solo performance.
The lawsuit claims that 'Thinking Out Loud' steals from 'Let's Get It On'.
"Thinking Out Loud" has become an global hit since it was released in 2016, earning Sheeran three Grammy nominations.
Last July, Ed reached an agreement to end a $20m copyright claim over his song "Photograph".
Structured Asset Sales is suing Sheeran for damages in excess of $100 million.
They claimed it was copied "note-for-note" from a song they wrote for X-Factor victor Matt Cardle.
Believe it or not, but "Thinking Out Loud" is not the only song of Sheeran's to be scrutinized in court.
In 2013, Gaye's children sued singers Robin Thicke and Pharrell Williams for copying their father's 1977 hit "Got to Give It Up" to create the hit "Blurred Lines", the Guardian reported.