Stack, who was capped by the Republic of Ireland last season, joins The Lions for the whole of the forthcoming campaign with Millwall securing a clause that states that he cannot be recalled by The Gunners.
The 23-year-old enjoyed a successful season at Highbury last term and played throughout Arsenal's run to the Carling Cup Semi-Final. He is set to become Millwall's first choice goalie after both Andy Marshall and Tony Warner left the club last week.