Do Mormons believe in Original Sin? Sort of...but not really. In only the most general terms can what Mormons believe about the fall of Adam and Eve be considered similar to what other Christian religions teach about Original Sin. Yes, Eve and Adam's individual choices to partake of the forbidden fruit brought mortality and 'fallenness' into the world.
But there the similarities end. Here are some of the major doctrinal differences:
--While other Christian religions teach that Adam and Eve rebelled against God, the Church of Jesus Christ of Latter-day Saints teaches that their ability to choose--and their resultant choices--were in accordance with God's plan.
--While others teach that children must be baptized in order to wipe away the stain of Original Sin, the doctrine of Christ's Church teaches that His atonement makes children innocent from the beginning.
--While other Christians teach that God's plan was thwarted by the Devil in getting Adam and Eve to partake of the fruit, LDS doctrine clarifies that Adam and Eve were not capable of having posterity until they became mortal and left the Garden of Eden.