The 30-year-old has played in several pre-season games and brings some much-needed defensive cover to the club ahead of their Champions League debut against Panathinaikos next week.
Ramsden, who turned down a new contract offer from Bradford, can play at centre-back, full-back or in a holding midfield role and looked comfortable on the ball during Motherwell's 1-1 draw with Everton on Saturday.
The former Grimsby and Rochdale player, who worked under Motherwell manager Stuart McCall at Bradford, is likely to start against Panathinaikos at Fir Park next Tuesday.
McCall lost centre-backs Tim Clancy and Stephen Craigan in the summer and both Jonathan Page and Steven Saunders have been ruled out of the third qualifying round first leg through injury.
Shaun Hutchinson is also a doubt for the visit of the Greeks after his pre-season was delayed by a groin problem and former Grimsby and Rochdale player Ramsden started alongside 19-year-old Adam Cummins against Everton.