Purpose: Mutant p53 stabilized by heat shock protein 90 (HSP90) is a novel target in oncology. The open-label, randomized phase II GANNET53 trial is the first to evaluate the HSP90 inhibitor ganetespib (G) with paclitaxel (P) in platinum-resistant epithelial ovarian cancer (EUDRACT 2013-003868-31; EU FP7 #602602). Patients and Methods: Patients were randomized 2:1 to receive G + P or P alone until progression. Primary endpoints were progression-free survival (PFS) and PFS rate at 6 months. Exploratory endpoints were biomarkers based on p53 and HSP90. Results: A total of 133 patients were enrolled. The median PFS was 3.5 (G + P) and 5.3 months (P) (HR ¼ 1.3; 95% confidence interval, 0.897–1.895; P ¼ 0.16), and PFS rates at 6 months were 22% (G + P) and 33% (P). No significant differences were found in overall survival, objective response rate, and post-progression PFS between arms. The most frequent adverse events were diarrhea (79% vs. 26%), anemia (46% vs. 51%), nausea (41% vs. 40%), and peripheral neuropathy (36% vs. 47%). Serious adverse events were more common in G + P (39.5% vs. 23.3%). Gastrointestinal perforation was a new safety finding. Despite a high TP53 mutation frequency, HSP90–p53 complexes were detected in only 39.6% of the cases and were also detected stably during treatment. In vitro, no synergistic effects of G + P were observed, and mutant p53 depletion did not sensitize ovarian cancer cells to treatment. Conclusions: Although no major safety findings were observed, G + P did not lead to survival benefit. Our companion diagnostic program confirmed that G + P do not favorably cooperate in killing ovarian cancer cells.