Background: Chemotherapy (CTX) becomes targeted therapy when biomarkers can predict a patient’s response. A relationship between resistance to taxanes and overexpression of class III b-tubulin (TUBB3) has been suggested by small clinical studies, but not confirmed in randomized trials. The Intergroup Trial of Adjuvant CTX in Adenocarcinoma of the Stomach (ITACA-S) evaluated the survival advantage of postoperative sequential CTX with FOLFIRI followed by docetaxel plus cisplatin in comparison to monotherapy with 5-FU/LV in patients with radically resected gastric cancer (N = 1106). The results showed no difference in survival between the two CTX arms. In a random subset of patients (N = 247) from the ITACA-S trial, we applied mass spectrometry-based proteomics to assess the role of TUBB3 as a predictive biomarker for response to taxane-containing therapy. Methods: Archived tumor tissues were microdissected and solubilized for proteomic analysis. TUBB3 and 44 other protein biomarkers were quantified using a mass spectrometry-based selected reaction monitoring assay. A predetermined TUBB3 cutoff of 700 amol/µg was based on the assay’s limit of detection. The Mantel-Cox log-rank test was used for survival comparisons. Results: Among patients treated with taxane-containing CTX (n = 125), those with TUBB3 levels below the cutoff had nearly twice the median overall survival (OS) as patients with TUBB3 levels above the cutoff (1566 vs. 801 days, p = 0.0282). TUBB3 levels made no statistical difference in survival among patients who did not receive taxane. Of note, among patients with high TUBB3 levels ( > 700 amol), those treated without taxane-containing CTX survived far longer than patients in the taxane arm (OS = 1991 vs 801 days, p = 0.048). Conclusions: Quantitative proteomic analysis of TUBB3 expression identified a subset of gastric cancer patients who benefitted from the addition of docetaxel to adjuvant CTX. Patients with high TUBB3 expression levels had worse outcomes on a taxane-containing regimen than on CTX without taxane. Personalized CTX based on the TUBB3 biomarker is promising and warrants further validation. Clinical trial information: NCT01989858