The 25-year-old Chilean international joins for a fee of just over £30million, having spent three seasons at the Nou Camp.
Sanchez cut his teeth in South America with Cobreloa, Colo-Colo and River Plate before making his name at Italian side Udinese.
He completed his medical this afternoon and will now become the club’s second most expensive player after Mezit Ozil.
“As many people have seen during the World Cup this summer, Alexis is
a fantastic footballer and we are delighted that he is joining us at
Arsenal Football Club,” Arsene Wenger told the club’s official site.
“Alexis will add power, creativity and much quality to our squad and
we are all looking forward to him joining up with us in a few weeks. He
has consistently produced top-quality performances at the highest level
for a number of seasons now and we are all excited to see him integrate
into the Arsenal squad.”