Jakob Ahlmann Nielsen (born 18 January 1991) is a Danish professional footballer who plays as a left back for AaB in the Danish Superliga. In August 2009, AaB signed Ahlmann from Midtjylland on a professional contract. He made his Superliga debut for AaB under head coach Magnus Pehrsson when he came on as a substitute on 16 May 2010, in the last match of the season, with 29 minutes left to play in a match against HB Køge.
The day after AaB won the Superliga, on 12 May 2014, Ahlmann was called up for the Denmark senior team for the first time by national coach Morten Olsen for the matches against Hungary and Sweden, along with fellow AaB players Kasper Kusk, Rasmus Würtz and Nicolaj Thomsen.