The 24-year-old is a first choice in Giovanni Trapattoni's World Cup qualifying team and could join Boro for a fee of about £3m in January.
He joined Preston for £225,000 from Peterborough three years ago and has made 139 appearances for them.
The Birmingham-born player has made 209 Football League appearances in total.
He could make his Boro debut in Tuesday's Championship match at Sheffield Wednesday.
St Ledger was Preston's player of the year in 2008 and will add competition to Middlesbrough's defence.
Boro's defensive record this season is one of the strongest in the Championship, having conceded only three goals in six games - while Preston have let in one more.
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