The 27-year-old, formerly of Manchester City and Everton, was a free agent after leaving Sunderland in the summer.
Rodwell played just six times for the Black Cats last season, with three appearances coming in the EFL Trophy for the club's Under-23s.
"It feels brilliant to sign here," the Englishman told the official Rovers website. "I've obviously had a bit of time off, but I can't wait to get going again. It's a great club and I'm just really excited to be playing again. Blackburn is a club which has won the Premier League before and it doesn't get bigger or better than that.
"After meeting the manager (Tony Mowbray), I feel it's an ambitious club and a great fit for me. I knew a bit about the club and he's told me a lot about the lads, the team spirit and the camaraderie in the group. That's something I will enjoy getting involved in. It's a new chapter and something I'm looking forward to.
"He sold the club to me and came across really well. I've played here a few times during my time at Everton and enjoyed playing at Ewood Park, so I'm looking forward to playing there in a Blackburn shirt now.
"I can't wait to get playing under him. It's important for me to get out there and start playing football again. There have been ups and downs over the years, but that's in the past and now I'm just looking forward and focused on the future ahead."
Rodwell won the Premier League title during a two-year spell with Man City before joining Sunderland in 2014, subsequently making just 76 appearances during an injury-plagued spell on Wearside.