Targeting NAD depletion in cancer cells has emerged as an attractive therapeutic strategy for cancer treatment, based on the higher reliance of malignant vs. healthy cells on NAD to sustain their aberrant proliferation and altered metabolism. NAD depletion is exquisitely observed when NAMPT, a key enzyme for the biosynthesis of NAD, is inhibited. Growing evidence suggests that alternative NAD sources present in a tumor environment can bypass NAMPT and render its inhibition ineffective. Here, we report the identification of nicotinaldehyde as a novel precursor that can be used for NAD biosynthesis by human leukemia cells. Nicotinaldehyde supplementation replenishes the intracellular NAD level in leukemia cells treated with NAMPT inhibitor APO866 and prevents APO866-induced oxidative stress, mitochondrial dysfunction and ATP depletion. We show here that NAD biosynthesis from nicotinaldehyde depends on NAPRT and occurs via the Preiss-Handler pathway. The availability of nicotinaldehyde in a tumor environment fully blunts the antitumor activity of APO866 in vitro and in vivo. This is the first study to report the role of nicotinaldehyde in the NAD-targeted anti-cancer treatment, highlighting the importance of the tumor metabolic environment in modulating the efficacy of NAD-lowering cancer therapy.