Badawi, 31, recalled that he was "surrounded by a cheering crowd who cried incessantly 'Allahu Akbar' (God is greatest)" during the whipping, according to a pre-released article from Der Spiegel's edition published Saturday.
Badawi received the first 50 of the 1,000 lashes he was sentenced to outside a mosque in the Red Sea city of Jeddah on January 9. Subsequent rounds of punishment were postponed on medical grounds.
"All this cruel suffering happened to me because I expressed my opinion," Badawi is quoted as writing in what Der Spiegel said was his first letter from prison since he was jailed in 2012.
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"He's in a poor condition," his wife Ensaf Haidar was quoted as saying. "He suffers from high blood pressure but above all he is mentally very stressed."
Saudi Arabia in early March dismissed criticism of its flogging of Badawi and "strongly denounced the media campaign around the case".
Badawi co-founded the Saudi Liberal Network Internet discussion group.
He was arrested in June 2012 under cybercrime provisions, and a judge ordered the website shut after it criticised Saudi Arabia's notorious religious police.
His case has sparked worldwide outrage and criticism from the United Nations, United States, the European Union, Canada and others.