Running back Ajayi, 26, who was born in London and moved to the United States aged seven, has been an avid gamer and football fan since his youth.
"Philadelphia holds a special place in my heart so I am blessed to be able to represent the city in the eSports realm and to become one of the first pro athletes to become a professional gamer," Ajayi said in a statement from the Union.
"It combines two of my passions, soccer and gaming. I'm a competitor, so you're going to see me give my all for the Union at every eMLS event this year."
The 2020 campaign opens Friday at Philadelphia, with eMLS entering a third season that features 25 FIFA20 players in three events -- League Series One, League Series Two and eMLS Cup.
The Union swept all three events last year thanks to "doolsta", who will play for Nashville this season.
"Signing Jay is another groundbreaking move in eMLS," Union marketing chief Doug Vosik said. "We're excited to see what he can do."
Ajayi played only three games for the Eagles in the just-completed NFL regular season, carrying 10 times for 30 yards before being released last month. His 2018 campaign was limited to four games by a torn knee ligament.