The 28-year-old goes straight into the Robins squad for this weekend's match against Aldershot.
Carter, who previously had spells with West Brom and Birmingham, agreed terms at the club's training ground on Friday morning.
"We're delighted. He has experience and a lot of quality," manager Mark Yates told the club's official website.
"It's known that I've been after a midfield player and patience has paid off.
"We're looking forward to getting him up to speed and getting him involved."
- West Brom