The 27-year-old right winger will go straight into the squad to face Mansfield Town at home on Saturday (3pm kick-off).
Haynes started his career in the youth set-up at Charlton, where he played alongside current Robins left-back Craig Braham-Barrett and the pair have remained close friends ever since.
He went on to play for Ipswich Town, Bristol City, Barnsley, another stint with Charlton and Notts County, along with loan spells at Millwall, Hibernian and Crewe Alexandra, making the vast majority of his appearances in the Championship.
He has also won caps at England Under-19 level.
* Winger Jamal Lawrence has joined Bishop's Cleeve on loan for a month, while youth team goalkeeper Harry Reynolds has also made the switch to Kayte Lane on work experience terms.