Butter 13X9-inch pan. Spread marshmallows and pecans evenly on bottom of pan.
Melt butterscotch chips in heavy, medium saucepan over low heat, stirring constantly. Stir in condensed milk until smooth.
Pour butterscotch mixture over marshmallows and pecans, covering entire mixture. If necessary, use knife or small spatula to help cover marshmallows and nuts with butterscotch mixture. Let stand in pan until set.