The micro-concert came Monday as Sheeran continued his testimony at his copyright infringement trial.
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Sheeran has been accused of copying the sheet music for "Let's Get It On" by the family of the song's late co-writer, Ed Townsend.
"Did you copy anything from "Let's Get It On" when you wrote "Thinking Out Loud?" defense attorney Ilene Farkas asked. "No," Sheeran replied.
"Were you thinking about "Let's Get it On?" Farkas asked. "No," Sheeran answered.
He testified his producers came to call "Thinking Out Loud" Sheeran's Van Morrison tune because of the similarities and influence Sheeran said the Northern Irish singer provided.
Sheeran also testified that he has "mashed up" his own original songs with other artists' songs in concert - something other musicians also do for fun and out of respect.
The musician also talked about artists ranging from Nina Simone, to Stevie Wonder, Dolly Parton and Morrison -- and weaving in the actual authentic song with his authentic song.
The chord progression and basic building blocks in Sheeran's song are frequently used, and didn't appear first in "Let's Get It On," Sheerhan's lawyer said earlier in the trial.
"Let's Get It On" has been heard in countless films and commercials and garnered hundreds of millions of streams, spins and radio plays since it came out in 1973. "Thinking Out Loud" won a Grammy for song of the year in 2016.
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The lawsuit was filed in 2017. The trial is expected to last up to two weeks.
Townsend, who also wrote the 1958 R&B doo-wop hit "For Your Love," was a singer, songwriter and lawyer. He died in 2003.
Kathryn Townsend Griffin, his daughter, is the plaintiff leading the lawsuit and insists the lawsuit is about protecting the authentic music.
"Intellectual properties now will be respected and doing the right thing is always the best," she said. "And I'm confident that God has got this whichever way it goes. I know that this statement will ring across the world. Let's get it on."
(The Associated Press contributed to this report.)