D'Arnaud, who recently described his play as 'unacceptable,' had been 3-for-26 since returning from disabled list for a concussion.
He is hitting .180 with three home runs and nine RBI in 145 plate appearances this season. He hit just .202 in 31 games with the Mets last season.
Triple-A C Taylor Teagarden will replace d'Arnaud on the roster.
Teagarden, 30, is hitting .279 with .402 OBP, 8 HR and 19 RBI with Las Vegas.
"I've just got work to do," d'Arnaud said (ESPN, June 7). "I've got to find my swing. That's all I really know. ... I don't know if I'm allowed to say anything, but like I said, I have work to do," he said.
David Wright later talked to d'Arnaud and told him to treat being demoted as an opportunity to get back on track.
"He's hit at every level in the minor leagues," Wright told reporters. "Now it's just a matter of going down there and getting at-bats where you don't have the pressures of performing here. ... He's got to remember that he's one of the best young catching prospects in baseball, and to go in with that mindset and, like I said, hopefully have some success at Triple-A and then be right back."