Background: Smoking by cancer patients and survivors causes adverse cancer treatment outcomes, but little information is available about how smoking can affect cancer treatment costs. Methods: We developed a model to estimate attributable cancer treatment failure because of continued smoking after a cancer diagnosis (AFs). Canadian health system data were used to determine the additional treatment cost for AFs for the most common cancers in Canada. Results: Of 206,000 patients diagnosed with cancer annually, an estimated 4789 experienced afs. The annual incremental cost associated with treating patients experiencing afs was estimated at between $198 million and $295 million (2017 Canadian dollars), reflecting an added incremental cost of $4,810–$7,162 per patient who continued to smoke. Analyses according to disease site demonstrated higher incremental costs where the smoking prevalence and the cost of individual second-line cancer treatment were highest. Of breast, prostate, colorectal, and lung cancers, lung cancer was associated with the highest incremental cost for treatment after AFs. Conclusions: The costs associated with afs in Canada after a cancer diagnosis are considerable. Populations in which the smoking prevalence and treatment costs are high are expected to benefit the most from efforts aimed at increasing smoking cessation capacity for patients newly diagnosed with cancer.