Ruddy, 22, is down the pecking order at Everton and moves to Gresty Road for a chance to impress the Goodison Park hierarchy.
It is the Crewe manager Gudjon Thordarson's second signing of the week as he desperately tries to claw his side away from the foot of the table.
Thordarson told the Stoke Sentinel: "I have wanted to strengthen in goal and I hope that I am bringing in a man who will do that for us.
"I know that John has done well in the league at other clubs. He will come in for a month initially and he trains with us today."
Ruddy arrived at Everton from Cambridge United for a fee of £250,000 in 2005. He has previously spent loan spells out at Walsall, Stockport County and Chester City.