On November 28, 2011, Arnaud was traded to new expansion team Montreal Impact for defender Seth Sinovic and allocation money. He was named captain of the Montreal Impact for the team's first season in Major League Soccer. On March 17, 2012, before a crowd of 58,912 at Olympic Stadium in Montreal, Arnaud scored the first ever goal for the Montreal Impact in MLS in the 56th minute by heading a cross from Sanna Nyassi into the top right corner against Chicago Fire keeper Paolo Tornaghi.