Simon Grayson said: "Lloyd is another good addition to our squad. He has plenty of experience, he has played at Championship level and is hungry and keen to do well. We're delighted to get things finalised and welcome him aboard before we go out to Slovakia."
Sam, who made 134 appearances for Charlton after working his way through the ranks at The Valley, is Leeds' sixth summer signing.
The Whites have a further 12-month option on the player, who was born in Leeds where his father went to university before moving to London.