28-year-old Robertson has signed after being released by releagted Chesterfield at the end of last season.
Edinburgh born Robertson started his career with Hearts in the youth setup before he moved south of the border and joined Nottingham Forest in February 2001. After progressing through the Forest academy Robertson went on to make 44 apperances, he then signed for Rotherham United in 2005 where in two years at Millmoor he made 56 apperances for the Millers.
In the summer of 2007 he joined Chesterfield where during five seasons made 116 appearances netting 3 goals in the process and made 23 apperances in the season Chesterfield got promoted in the 2010-2011 season.
Last season Robertson made 14 apperances for the Spireites in a injury hit season.
Robertson told the official website "I was keen to stay in League One," he added.
"The facilities were fantastic, and I spoke to people I know who were here last season and they were full of praise for the management staff.
"There was the ability of the team as well, they were all big factors for me."