The 31-year-old, who has 102 caps for the Lionesses, joins Juve having spent the past six years at English champions Chelsea, following an earlier spell with them between 2007 and 2009.
Aluko announced last month she would leave the Blues at the end of the 2017-18 campaign.
She scored seven goals in all competitions last season as Chelsea capped a domestic double by going unbeaten in the league and winning the FA Cup. They also reached the semi-finals of the Champions League.
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Juventus only formed their women's senior team last year but won the Serie A title in their debut season and will compete in next season's Champions League.
Aluko was part of the England squad that finished runners-up at the 2009 European Championships and also achieved third place at the World Cup in Canada three years ago.
However, she has not been selected for the national side since May 2016 and was involved in a discrimination case against former England boss Mark Sampson last year.
(Reporting by Christian Radnedge; Editing by Neville Dalton)
