The fan, identified by The Philadelphia Inquirer as Joie Henney, arrived at Citizens Bank Park walking Wally on a leash, claiming the nearly 5-foot alligator as his service animal.
The stadium's policy on support animals on the Phillies' official website states: "Certified service dogs or service dogs in training for guests with special needs are welcome. All other animals are prohibited."
Photos and videos posted to social media showing Henney outside the stadium with Wally went viral Wednesday night.
Accounts linked to Wally have a growing following on social media; the Instagram account @wallygatornjoie had nearly 26,000 followers as of Thursday morning, while the TikTok account wallythealligator had over 100,000 followers.
Wally's denied entry drew comparisons to the famous "Curse of the Billy Goat" involving the Chicago Cubs. In 1945, William Sianis put a curse on the Cubs when he and his goat were denied entry into a World Series game at Wrigley Field.
Outraged at not being able to watch his beloved Cubs, Sianis placed the curse on the team, saying the Cubs would never again win a World Series.
Chicago would not play in another World Series for 71 years, but the curse finally was broken in 2016 when the Cubs celebrated their first championship in 108 years.
Information from Reuters was used in this report.