The versatile 27-year-old, who is comfortable playing at full-back or in the centre of defence, has agreed to join Pools on a loan deal which runs until January 2018.
Watson made his name after coming through the ranks at Dundee United where he made his senior debut at just 17 and went on to make over 150 appearances during an eight-year stay at Tannadice.
He joined the Saints last summer from St Mirren, played in six competitive matches under Tommy Wright last season and scored one goal.
Pools boss Harrison has made his move to bring Watson south of the border for the first time his career following the injury to Louis Laing and with Liam Donnelly set to link-up with Northern Ireland Under-21s next week.
Hartlepool will now work to secure international clearance in time for him to feature in tomorrow’s game against AFC Fylde.