The midfielder, 25, spent the second half of last season at Roma having fallen out of favour under Mikel Arteta and spent this summer available for another move.
On Thursday, Southampton confirmed the deal for Maitland-Niles until the end of the campaign.
“Ainsley is at a good point in his career, with a lot of experience already, but he is also still a player with potential, and he is someone who can definitely help us,” said Saints boss Ralph Hasenhuttl.
“I think he fits well with us and can develop his game even further during his time here. He can also play in different positions, which is always a good thing, and I am pleased to welcome him as part of our squad.”
Arsenal have also completed a loan exit for Miguel Azeez to Spanish second-tier side UD Ibiza. Hector Bellerin is also nearing a move to Barcelona.