The former Preston North End left-back will team-up with his new Harrogate colleagues once he has completed international duty.
He misses the home game with Crawley Town in League Two.
Slater played for Republic of Ireland under-21s as they won 2-1 in Moldova in a Euros qualifier.
Albion under-21s boss Shannon Ruth told The Argus in early August that Slater was one of the players for whom the club were seeking the right EFL loan move.
Lloyd Kerry, who is head of recruitment at Harrogate, identified him as a good fit for the Yorkshire club.
Harrogate boss Simon Weaver said: "Jacob has an impressive background and is a young, athletic left back.
"We're excited about adding to the squad with more quality to choose from.
"He came up late, to be honest, but Lloyd had been aware of him for some time.
"When you get bought for big money at a young age, there's big potential.
"He's got a good left foot, strides forward well and has got a bit of an edge, which we like in all positions.
"We feel he can suit our style of play, which is massively advantageous if we can put people in the team and it's not removed from what we are doing right now."
Mark O’Mahony, the Albion striker on loan at Reading, and Jamie Mullins, the under-21s midfielder who moved on to Wycombe in a permanent deal this summer, also started for the Irish in Moldova.
They continue their qualifying campaign when they host Andorra in Dublin on Tuesday.