The 25-year-old striker was down the pecking order at West Brom, who have agreed to let him leave.
Beattie, who spent time on loan with Crystal Palace earlier this season, has signed in time to be considered for Saturday's clash at Plymouth.
Blades manager Kevin Blackwell was delighted to land Beattie.
"Craig is a Scottish international and a player with a proven track record at this level," said Blackwell.
"He gives us much-needed reinforcement for the run-in."