FAYETTEVILLE, Ark.—Wal-Mart Stores Inc. sought to reassure investors Friday that its shares remain a compelling value despite two straight years of slumping U.S. sales by disclosing a new $15 billion share buyback plan.

With Wal-Mart's shares stuck in a rut, the discount retailer has been returning more of its excess cash to shareholders by buying back shares. Last year, it acquired $13 billion of its shares. The purchases can increase per share profit by reducing total shares outstanding, a strategy employed by numerous retailers...