Which What If...? issue first featured Mayday Parker, later known as Spider-Girl?

Mayday Parker, later known as Spider-Girl, first appeared in What If...? #105.

Published in February 1998, the issue imagined an alternate future in which Peter Parker and Mary Jane Watson had a teenage daughter. Mayday inherited her father's spider-powered abilities and eventually adopted the Spider-Girl identity. The story introduced Marvel's MC2 universe, later designated Earth-982.

The character was created by writer Tom DeFalco and artist Ron Frenz. Strong reader interest led Marvel to launch Spider-Girl as an ongoing series, where Mayday balanced superhero responsibilities with school, family life, and the legacy of her famous father.

A common mix-up is confusing her debut with the start of her solo title. What If...? #105 introduced Mayday; Spider-Girl #1 came afterward as part of the MC2 line. She later appeared in Spider-Verse and other multiverse stories, and an infant version appeared in Spider-Man: Across the Spider-Verse.