Spratt, 19, started his career at Manchester United before joining up with Huddersfield Town upon his release.
After leaving the Terriers, Spratt has since been plying his trade with York City, before joining Boro on a one-month loan deal.
Delighted boss Darren Kelly said: "Harry has a good pedigree, we're delighted to have him on a one month loan.
"He's a very good left sided defender, another player that will make us stronger."
Spratt joins Kieran Glynn, Jacob Gratton and Ryan Blott in joining Boro during a busy week.