"I am looking forward to working with Alex," said Pars manager Jim McIntyre.
"If you saw the semi-final and final of the Scottish Cup, I am sure you will agree that he will bring strength in depth to our defence."
A former FA Youth Cup winner with Leeds United, left-sided Keddie has been a key figure for County manager Derek Adams.
He made several vital blocks and challenges in his club's cup final appearance on Saturday, though could not prevent Dundee United lifting the trophy after a 3-0 win.
Source: BBC Sport
Source: BBC Sport