The 23-year-old free agent had been with Northern Premier League Premier Division side Warrington Town since a brief early season stay at League Two side Accrington Stanley.
He left Scottish Championship side Raith Rovers to move to Accrington.
The ex-Ipswich trainee had two seasons at Raith, making 87 appearances.
He also played north of the border for both Kilmarnock and Cowdenbeath.
McKeown, who can play as a full-back, wing-back or winger, will be available for the New Year`s Day home fixture against Wrexham, who beat the Sandgrounders 1-0 on Boxing Day.
Steve Burr's Southport, who are 21st in the National League, hope to make a further loan signing in time for the return game with fellow strugglers Wrexham.
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Southport have signed former Northern Ireland Under-21 utility player Rory McKeown to aid their battle to escape relegation from the National League.