Borlée missed most of the 2014 season, bar a relay outing at the 2014 IAAF World Relays, where he was ninth. He returned in strong form for the 2015 indoor season, winning his first national title at the Belgian Indoor Championships. A personal best of 46,73 seconds at a meeting in Metz earned him a place at the 2015 European Athletics Indoor Championships. In his first individual outing for Belgium, he ran a personal best of 46.72 seconds in the semi-final before improving to 46.25 seconds in the final – a mark which was enough to take the silver medal in Prague behind the clear home favourite Pavel Maslák. A day later he won the gold in the 4 × 400 metres relay, along with his brothers and Julien Watrin, in a new European indoor record of 3:02.87.