She began her career at Hamburger SV in 2021 and helped the German club secure two promotions in three years to reach the top division.
Baum made 68 appearances, scoring 21 goals in total for Hamburger before joining RB Leipzig last summer.
At international level, Baum has represented Germany’s youth age groups, rising from the under 17s to under 23s.
She will wear the number 19 shirt at Arsenal and becomes the club’s fifth signing this summer.
“Lisa is a young wide forward who will bring direct intent and athleticism to our attacking group,” said manager Renee Slegers.
“At 19, she is still developing and we believe she has huge potential, which we will support her to achieve. We welcome her to Arsenal and look forward to the road ahead together.”
