Lothar Sieber (7 April 1922 in Dresden, Germany – 1 March 1945 near Stetten am kalten Markt, Germany) was a German test pilot who was killed in the first vertical take-off manned rocket flight, in a Bachem Ba 349 "Natter".
Before he became a test pilot for Bachem, he piloted an Arado Ar 232 in highly risky sorties.
Shortly before his flight, he had become engaged to Gertrud Naudit, a Luftwaffenhelfer.
Sieber had held the rank of second lieutenant but was demoted to private after an alcohol-related AWOL.