Lothar Sieber (7 April 1922 – 1 March 1945) was a German test pilot who was killed in the first vertical take-off manned rocket flight, in a Bachem Ba 349 "Natter".

On 1 March 1945, at the Truppenübungsplatz Heuberg, Sieber entered the Natter Ba 349A M23 for the first manned vertical take-off of a rocket.

It is estimated the Natter reached 1,500 m (4,900 ft), at which point it nose-dived and hit the ground with great force about 32 seconds later, some kilometres from the launch site.

[5] The pilot's headrest had been attached to the underside of the canopy and as the canopy flew off the pilot's head would have snapped back suddenly about 25 cm (9.8 in), hitting the solid wooden rear upper cockpit bulkhead, and either knocking Sieber unconscious or breaking his neck.

In 1998–1999, excavations found the remains of one of the Starthilfsraketen RATO rockets at the impact site, proving that it did not release from the fuselage of the Natter.