The 28-year-old had been training with the Seasiders for the past two weeks, having previously played under boss Gary Bowyer while on loan at Blackburn Rovers.
Spearing is a free agent, having been without a club since failing to agree a new deal with Bolton Wanderers in the summer.
The Wallasey-born player turned out 55 times during his time at Anfield, before going on to make 173 appearances for Bolton.
"I've been speaking to the manager throughout the summer about a few bits and bobs and he's given me the confidence to come in and train, as well as the time to try and get fit, get sharp and get ready," Spearing told the official Blackpool site.
"Thankfully we got a deal sorted and I'm now ready to kick on. The standard of the lads in training has been brilliant and the gaffer has got them playing in the right direction."
Blackpool are ninth in League One and face an away game at Walsall on Saturday.