It starts in Loch Tay , and opens up into North Sea at Firth of Tay . The Tay originates in western Scotland on the slopes of Ben Lui ( Beinn Laoigh ) , then flows easterly across the Highlands , through Loch Dochhart , Loch Lubhair and Loch Tay , then continues east through Strathtay ( see Strath ) , in the center of Scotland , then southeasterly through Perth , where it becomes tidal , at its mouth at the Firth of Tay , south of Dundee .