Officer William Kelly of Norfolk, Virginia, donated to Rittenhouse, The Guardian reported last week.
Rittenhouse is accused of fatally shooting two people at an anti-racism protest last August.
Norfolk officials announced Kelly's firing on Tuesday, saying his donation eroded public trust.
A Norfolk, Virginia police officer has been fired for donating to the legal defense of Kyle Rittenhouse, the 17-year-old accused of fatally shooting two anti-racism protesters last year.
Rittenhouse is accused of killing two protesters and injuring a third after he opened fire at a demonstration against the police shooting of Jacob Blake, a Black man in Kenosha, Wisconsin, last year.