The 28-year-old Argentine, who has signed a four-year deal, helped Arsenal to FA Cup victory last season and Community Shield glory last month but faced playing second fiddle to Bernd Leno.
Aston Villa sign Arsenal goalkeeper Emiliano Martinez
Aston Villa announced the signing of Arsenal goalkeeper Emiliano Martinez on Wednesday for a reported fee of £20 million ($26 million).
Villa boss Dean Smith said: "We are really pleased with the signing of our new goalkeeper Emiliano Martinez.
"We know how highly Arsenal rated him and we watched his outstanding performances last season in a trophy-winning top side.
"We moved for Emi when we saw the opportunity, as it is rare to be able to buy a top-class goalkeeper who hasn't yet reached their peak age and who can therefore be a key player for our club for the long term."
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