The agreement, which sees the left-back switch to The Hawthorns, does not include a recall clause, meaning the England youth international will remain there until the end of the 2016/17 season.
Loan move important to gain Premier League experience
Everton manager Ronald Koeman has allowed Galloway to leave on a short-term deal having already bolstered his central defensive options with Ashley Williams and the Blues also remain hopeful of signing Lamine Kone from Sunderland.
The Blues boss believes the left-back's development is best served away from Goodison Park in order to gain first-team experience in the Premier League with Leighton Baines Everton's first-choice left-back and Koeman believing Bryan Oviedo provides adequate back-up.
The 20-year-old is highly regarded and signed a five-year deal with the Toffees in August 2014 after joining from MK Dons. Galloway impressed for the Toffees in their Under-21 ranks and was handed with his senior Premier League debut in May 2015 when he started a 2-1 win at West Ham United.
Galloway firmly made his first-team breakthrough last season, starting 14 Premier League matches and he was rewarded with an improved contract earlier this summer. He was an unused substitute in their 1-1 draw with Tottenham Hotspur on the opening day of the season, before he was left out of the squad for Saturday's match against his new club.
Galloway the latest Blues player to leave on loan
Galloway is the fourth Everton player to be sent out on loan, joining Luke Garbutt, Conor Grant and Russell Griffiths who have moved to Wigan Athletic, Ipswich Town and Cheltenham Town respectively.
Garbutt has made just five appearances for the Toffees in the Premier League since joining the Blues in 2011 after struggling to find a path into the first-team ahead of Baines, Oviedo and Galloway. The 23-year-old spent last season on loan at Fulham and he will be hoping to impress at Wigan during his loan spell until January 2.
Conor Grant is another player who will be hoping to impress while on loan at Ipswich after signing a new two-year deal with the Blues last month to keep him at Goodison Park until the summer of 2018. The 21-year-old spent a loan spell at Motherwell in 2015, before making 22 appearances whilst on loan at Doncaster Rovers last season.
Like Grant, Goalkeeper Russell Griffiths signed a contract extension last month, but he will be continuing his development at Cheltenham Town until January 2. After taking part in pre-season with Northwich Victoria in 2015, the 20-year-old had a loan spell with the Northern Premier League side, before another at National League Halifax Town.