"I'd not really been getting a look-in at Palace, I needed to play some games and the manager here wanted a midfielder so it worked out well," he told Plymouth's official website.
"Hopefully we can help each other out. I want to help get some results while I'm down here and then we'll see where it goes."
Argyle boss Paul Sturrock added: "We needed another captain in there and he has captained Wales and many other teams in the past, so there are leadership qualities in him."
Fletcher will make his debut at home to Sheffield Utd on Saturday.