She launched her senior career with Hamburger SV in 2021, helping the German side achieve two promotions in three years to reach the top tier.
Before moving to RB Leipzig last summer, Baum made 68 appearances for Hamburger SV, scoring 21 goals.
Internationally, Baum has progressed through Germany’s youth ranks, from the under-17s up to the under-23s.
She will wear the number 19 shirt at Arsenal and is the club’s fifth signing of the summer.
Manager Renee Slegers described Lisa as a young wide forward whose direct style and athleticism will strengthen the attack.
Slegers added that at 19 she is still developing and has significant potential, which the club will help her realize, welcoming her to Arsenal and looking forward to the journey ahead.