The 94-year-old retired justice tells NPR that public opinion has changed on the issue.
Stevens also says that there isn't much distinction between marijuana and alcoholic beverages. He says that the prohibition against alcohol in the early 20th century is generally thought not to have been worth the cost and that he believes that will be how marijuana is viewed in the future.
Stevens is the author of a new book, "Six Amendments: How and Why We Should Change the Constitution," in which he proposes banning capital punishment and limiting gun rights.
NPR will air its interview with Stevens on the program "Weekend Edition Saturday."