He started his professional career with Borussia Dortmund, making his first appearance on 9 August 2008, in the German Cup first-round match against Rot-Weiss Essen.

[4] His first league appearance came in the following week, as he played the final twenty minutes of a 3–2 win at Bayer Leverkusen.

[5] In addition to being the left back for the team for the entire season, Schmelzer also helped Dortmund secure the DFB-Pokal with a 5–2 win against Bayern Munich, making it three titles in two years.

[9] On 31 May 2020, he scored his first Bundesliga goal in seven years in a 6–1 win against Paderborn, coming off the bench for Moroccan right-back Achraf Hakimi.

[13] On 3 May 2024, it was announced that he would join Jan Zimmermann's coaching staff at Borussia Dortmund II for the 2024–25 season.