The 31-year-old had recently spent a trial period with Swindon Town following his release by Southend United, but has opted to try his luck at Boundary Park.
The former Tottenham Hotspur trainee had spent just over a decade at Roots Hall, but fell out of favour last season and spent the last two months of the campaign on loan at Gillingham.
Maher made more than 400 appearances for the Shrimpers in total, being instrumental in their rise to the Championship for the 2006-07 season.