The 31-year-old musician shared his picture with the former First Lady on Instagram, where she can be seen holding the T-shirt with 'OBAMA 24k' embossed on its back.
"Last Night in DC Mrs Obama came to the concert and blessed us with her presence #24kmagicworldtour #Hooligans," wrote Mars, alongside the picture.
Michelle, who is a fan of the "That's What I Like" singer, attended his concert here.
Mars also shared another photograph of Obama, with his band, Hooligans and captioned it as, "#squad".