Letheren played as first-choice goalkeeper for Dundee throughout all of the 2013–14 season. He was voted supporter's association Player of the Year after pulling off a match-winning save in the dying moment of the final game of the season to secure Dundee the Scottish Championship title. Letheren played the first ten matches at the start of the 2014–15 season before injuring his knee in a warm up against Motherwell. It was later announced that he would be out until December. After making his return against Partick Thistle and St Mirren, Letheren was once again sidelined after suffering an injury in the warm-up ahead of the Dundee derby against Dundee United, which allowed Arvid Schenk to take his place, although Schenk went on to concede six goals, in a 6–2 loss. Though he recovered from his injury, Letheren lost his first choice goalkeeper status to Scott Bain throughout the season. However, Letheren was given a chance to play the last three matches at the end of the season following Bain's injury.