Alli has signed an 18-month contract with the Italian club, with an option to extend for an additional 12 months.
The 28-year-old could make his debut against Udinese on Monday (Tuesday AEDT).
"The club believes in Dele's potential and is committed to helping him rediscover his best form," Como coach Cesc Fabregas said.
"His experience and leadership qualities will undoubtedly benefit the squad."
Alli was one of English soccer's biggest talents, a member of the England team that reached the World Cup semi-finals in 2018 after a period when he was a star of Mauricio Pochettino's young Tottenham side that nearly won the Premier League.
However, the attacking midfielder's career has derailed in recent years and he spent six weeks in rehab in the US in 2023 in a bid to deal with mental-health problems stemming from a traumatic childhood and an addiction to sleeping pills.
Alli did not play at all last season for Everton before his contract expired in June.
His last appearances were during a loan spell at Turkish club Besiktas the previous campaign.
"The club recognizes (sic) his exceptional talent and is excited to bring him into the team," Como said.
"The focus will be on providing Dele with a supportive environment where he can gradually integrate into the squad.
"While there will be no immediate performance expectations, the club is confident that he will make a significant contribution both on the pitch and as a mentor to the club's young talents."
Como are 17th in the 20-team Italian league.
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