The 23-year-old was keen to secure more game time ahead of this summer's Euros, having started just four Premier League games for the Gunners this season.
Maitland-Niles looked set to join Southampton earlier on deadline day, but the Saints pulled the pull which allowed the Baggies to swoop in to complete the deal.
Speaking about the deal, Mikel Arteta said: “Ainsley is a very talented player and we are proud of his hard work and development with us.
"This decision to go on loan to West Bromwich Albion for the rest of this season is to support Ainsley and his growth further.
"He will be working at a very good club in the Premier League and we all wish him the best of luck during his time at the Hawthorns."
Technical director Edu added: "We wish Ainsley the best of luck for his time with West Bromwich Albion.
"He deserves the opportunity to play consistently, which is what he needs for his club and international career ambitions and we fully support him with that."
Maitland-Niles' loan move does not include an option to buy.
The 23-year-old joined the Arsenal Academy aged six, where he progressed through every age group and has now made 121 appearances for the first-team.